Adductor magnus is one of the few "internal" rotators of the hip. (There seem to be lots more external hip rotators than internal hip rotators!)
It does so by pulling rearwards on the inner thigh, just above the knee.
Because it runs up the inner thigh from just above the knee to the sitting bone, it can also be used to pull the thigh inwards (adduction.)
A quick note on Activating the Adductor Magnus.
I first learned by trying to create space in my hip joint. Since the space creators are all external rotators, either the adductor magnus or gluteus minimus or TFL will activate to counter this and keep the leg from rotating.
By practicing "reaching" my knee away from my hip joint I learned to feel adductor magnus activating. Pushing the inner thigh back (standing) or down (sitting with legs straight) produces the same sensation.
Hence one way to deliberately activate adductor magnus is to press the inner thighs back or down.
It can also be used to prevent external rotation.
In so doing it can be working against external rotators to keep the thigh from rotating, or it may be working against the weight of the thigh itself to prevent external rotation.
As an example of the latter case, with the leg lifted rearwards such as in extended cat pose, the feeling is like the outer thigh is hanging from the inner thigh (or more precisely, the inner knee.)
It can also be used to "extend" the hip:
It's often called the extra hamstring because like the hamstring muscles it runs from the sitting bones, running down the back of the thigh to attach to the inside of the femur, just above the knee. The difference is that the hamstrings attach below the knee to the tibia and fibula. Instead of operating on two joints (the hip and the knee) the Adductor Magnus only operates on one, the hip.
In a belly facing down yoga pose like extended cat pose, you can use the adductor magnus to lift the leg and keep the knee pointing down with minimum effort.
It can feel like the leg is hanging from the inner knee.
Rather than using the gluteus maximus to lift the leg, or rather than just using the gluteus maximus, you can use adductor magnus to keep the leg suspended and even to lift it higher.
And so that the knee points down naturally, due to the weight of the leg, you can pull up on the inner knee and thigh to activate adductor magnus while at the same time letting the weight of the outer thigh hang down.
It may be that because the leg is horizontal the adductor magnus is better suited because it's point of attachment is closer to the legs center of gravity than say the gluteus maximus.
As a result I find it easier to lift the leg (while still keeping the knee pointing down) by focusing on pulling up with the inner thigh.
This could be considered a back bending type action, particularly if you lift the leg above the horizontal. Personally I used to instruct bending the spine backwards more to allow the leg to lift higher.
Starting with the leg horizontal, from there I'd suggest tilting the pelvis forwards and curving the lumbar spine backwards (or upwards in this case) to lift the leg higher. It would then be mostly the spine doing the backbend. The leg would then stay stationary relative to the pelvis. The pelvis and leg would tilt forwards together, as one unit.
Another option is to keep the lumbar spine straight using the abdominals and focus on moving the leg relative to the pelvis (rather than tilting the pelvis forwards and moving the leg and pelvis as one unit.) In this case the "backwards bending" takes place more at the hip helping to open the front of the hip joint.
I'd suggest that the ideal is learning to do both while at the same time feeling your hip, lumbar spine and waist at the same time. You can then choose to bend backwards mostly at the lumbar spine, mostly at the hip or using a combination of both.
In all cases, Adductor Magnus can be used to pull upwards on the inside of the knee, either to keep the leg lifted and in place, or to lift the leg higher.
In these two poses you lay on your back and then with feet on the floor, use your legs to push the pelvis upwards. When pressing the pelvis up, focus on pressing the inner thighs down. You aren't actually pressing the inner thighs down, but pressing them down may be one way of activating the adductor mag, and it in turn may help you to lift your pelvis higher.
I'd suggest in any yoga pose you work to avoid any feeling of compression or "density."
Using the inner thighs (or learning to use them) gives you a tool you can play with towards that end.
If you are using your butt muscles too much, or if your butt feels jammed or too tight, try activating the adductor magnus as well to see if it helps. This isn't to say that you should try to deactivate the Gluteus Maximus, but instead help it out.
Because the adductor magnus is an internal rotator, activating it in bridge and wheel pose may help you to keep your knees pointing in the same direction as you toes. But prior to lifting up, I'd suggest that you set your feet up so that they are parallel.
It can seem odd to press your inner thighs down while trying to press your pelvis up. Think of it as a way of pressing your knee down. The natural response then, as your knee (and shin press down into the floor) is for the pelvis to press up.
More importantly, while pressing your pelvis up (using whatever means necessary) notice if your pelvis goes higher when pressing the inner thighs down. Or notice if this "activation" makes pressing your pelvis up easier.
There are other actions that can be done in concert with pressing down with the inner thighs which I'll talk about in the article on the SI Joint (which may still have to be modified.)
One other backbending posture where the Adductor Magnus may be useful is in Camel Pose.
In extended cat pose you lift one leg and move the leg backwards with respect to the pelvis (or actually, upwards.) In wheel and bridge you used the legs to push the pelvis upwards. Because the feet are in the floor in both of these poses, the result is that the pelvis moves up relative to the legs (in a standing position, the pelvis would equivalently move forwards.)
One way of entering came pose is to start in a kneeling position with your hands on the floor behind you.
Starting with your shoulders:
As in wheel pose and bridge pose, push the inner thighs down (and slightly back) while pushing the pelvis forwards and up.
In camel pose, you may find it comfortable to have your shins about hip width (or maybe even shoulder width) apart. Use a position that is comfortable given what you are trying to do.
Although I am not expert at dropping back (it's one of those actions that I had for a while and then didn't do for awhile and then got too scared to actually do the drop back) when I was doing drop backs I did them with my feet turned out alot. I'd collapse into my arches which made it easier to push my knees forwards so that then I could get my center over my feet. This made it easier to drop back and land on my hands and also to pull back up to standing.
I recently tried to do a standing back bend (without actually dropping back) and found that pushing the inner thighs back while pushing my pelvis forwards allowed me to bend back fairly deeply. However another component of this was pulling the sitting bones inwards and the tailbone forwards, as if bringing the tips of three fingers together.
Although I've talked mainly about activating the adductor magnus by pushing the inner thigh back (and it's my assumption that its the adductor magnus that is activated by doing this "action") There are also times where I've experimented with pushing the inner thigh forwards.
While this may not be the adductor magnus that is activating, it's good to be aware of this potential action. I've found that if I'm not sure what to do, I try both and see which works best.
One other area where pressing the inner thigh back or forwards may be helpful is when doing one legged balancing poses. I've found that when standing on one leg pressing the inner thigh forwards (half moon pose) or backwards (warrior 3, tree pose, eagle?) helps to make the standing leg more stable. And it also helps when standing up from triangle pose (press the inner thigh of the front leg "forwards.")
In the hip joint article I talked about creating space in the hip joint using the obturators and the gemellus, all of which are external rotators. I also suggested that this external rotation is probably counted by the Adductor Magnus to keep the thigh from rotating. (possible the TFL and gluteus minimus as well.)
It may be that if you press your inner thigh back a natural response is for the obturators and gemellii to activate and so activating the adductor magnus may automatically help to create space in the hip. But if it doesn't, you could experiment with activating adductor magnus (by pressing down with the inner thigh, or back if you are standing) while at the same time making your thigh feel long. Sitting upright with legs straight ahead you could focus on pushing the inner thighs forwards and down to activate both sets of muscles. You can try using this action in any forward bending hamstring stretch.
So that you can lengthen your hamstrings with greater ease you can also experiment with engaging your buttocks at the same time so that they take the strain of supporting your upper body. You can then gradually tilt the pelvis forwards using the gluteus maximus as the controlling muscle.
Note that this can feel like a "spreading" or in Richard Freemans' words a "flowering or blossoming" of the buttocks. (actually those aren't his words, but I'm sure he said something like that, while at the same time reminding me of a bushy eyebrowed agent smith from the matrix.)