This sequence of virasana (hero pose) variations will hopefully help you work towards getting your bum to the floor between your feet and then reclining, gradually, safely and with minimal discomfort.
In virasana, you knee with the top of each foot flat on the floor. This stretches the front of the ankle and the top of the foot.
The shins are folded to the outside of the thighs which stretches the inside of the knee. This position, with the butt on the floor, also creates a stretch for the quadriceps.
There is also a slight inward rotation of the thigh at the hip joint. Because of this internal rotation, hero type poses can be used as a counterpose or balancing action for poses where the thighs are externally rotated.
Lotus position is one of the main "externally rotated" leg positions that hero pose balances.
When I first saw a picture of Virasana, the upright version, I thought I'd never be able to get my butt to the floor between my feet. At the time I was just barely comfortable with my bum resting on my heels. I've since met people who can't even get their bums to their heels.
This can make doing virasana (or hero pose) a little bit difficult.
Hero pose stretches the front of the thighs, the quadriceps. The reclining version of this pose,supta virasana stretches the quadriceps even more deeply and possibly stretches the psoas and rectus femoris also.
Depending on how you do the reclining version, you can also use it to strengthen the abs. Because this pose opens the front of the knees and hips, it can be considered a back bending yoga pose.
One way to make hero pose easier is to do it one leg at a time.
The easiest version is to have one leg in hero pose and the other foot close to the inner thigh in the "janu sirsasana" foot position.
You can then practice pushing your janu sirsasana leg into the floor at the same time sitting up tall. As you sit up tall push your hero leg hip downwards towards the floor. For extra leverage try sidebending towards the hero leg side.
You can also try twisting towards the hero leg side.
Rather than holding the pose, I'd suggest move in and out of the side bend or twist with your breath. Inhale and lengthen and push your hero leg hip down. Then exhale and relax.
This next virasana variation is called "Triang Muka Eka Pada Paschimottanasana" in the Ashtanga Yoga series.
Sit with one leg straight and the other shin folded to the outside of the thigh in hero position. Initially start upright. If your butt on the hero leg side doesn't touch the floor then slowly sit up tall and use your body weight to help sink that hip. Then relax. Repeat.
If you are synchronizing movement and breath I'd suggest sitting up tall as you inhale and relaxing as you exhale.
You can also reach your arms up as you inhale. (Try in with hands apart and also with fingers interlaced and palms facing upwards. In this case each time you lengthen focus on pushing your palms upwards. Relax by bending the elbows slightly and let your shoulders move down.)
For leverage to sit up tall, press your straight leg down into the floor.
Next, bend forwards. Have your hands on the floor to begin with. Here again work on lengthening your spine. But so that the hip on your hero leg side sinks down, press your straight leg down into the floor.
Each inhale lengthen your spine and press your straight leg down. Each exhale relax and let you ribcage and head sink down.
If you want to grab the foot with both hands, or grab a wrist behind the foot, then use your straight leg for support and push it down into the floor.
You can also think of this pose as a hamstring stretching yoga pose.
Next, sit up tall again and if you can get your hero leg hip close to the floor, try laying back. If you like try laying back on some sort of support. Then gradually reduce the height of the support. This is where the quadriceps stretch can become a little bit more intense.
When laying back in half hero pose you may find it helpful to lift your hips/pelvis and move it forwards or back, left or right relative to the bent knee. The idea is to find a position that is comfortable for your hero leg knee and hip.
(For variations of this check out Bent knee hip flexor stretches. It includes tips on how to activate your foot or your hip to make the stretch more effective.)
Once you do find a comfortable position lay down fully (if possible.) Focus on moving your hero leg knee towards the straight leg knee. Also focus on lengthening your spine. Reach your head and ribs back away from your pelvis. You may find it helpful if you focus on moving the backs of your ribs away from your pelvis.
When working on the full upright version of hero pose, you can start with your feet together. If you bum doesn't touch your heels, try sitting upright while inhaling, then bent forwards to allow your hips to lift while exhaling. (Put your hands forwards on the floor if you need to.) Rather than bouncing, try working slowly and smoothly.
If you can get your bum to your heels, the next step is to widen your feet to give room for your bum to sink to the floor between your feet. Rather than turning the feet out (which is a different yoga pose) have the tops of your feet on the floor.
You can either stay sitting upright, or try sitting upright (inhale) and lean forward and relax the tension while exhaling.
You can also try sticking books or blocks under your butt and then gradually reduce the height of your prop.
Once your butt gets reasonably close to the floor (perhaps you feel the skin of your butt touching the floor but not your sitting bones) you can work at leaning back.
When I first started doing the reclining down version of this pose with both knees bent, I'd feel a strong pulling sensation near the bottom of my lower back. I'm pretty sure that what I was feeling was the lower fibers of my psoas major muscle.
As such this pose can either stretch the lower fibers of the psoas muscle or it requires you to relax those fibers in order to stay in the pose comfortably with that pulling sensation on the bottom of the lower back.
Note that the reclining version may actually be injurious since the lower fibers of the psoas are pulling forwards on the lumbar spine and the upper fibers are not. This creates a shear force that tries to pull the upper and lower lumbar spine apart.
The way around it is to try to activate the abs and lengthen the back ribs away from the pelvis at the same time. You can also try squeezing your inner thighs together. This may allow the psoas to shorten and so release the pull or at least reduce it.
Note, when doing hero yoga pose it might feel easier on the knees to do it with the knees apart. And that's the way I've done it for the last few years. However, as I recently learned, if you bring the knees together, you reduce the pull on the psoas muscle. (The psoas is a hip flexor.)
And if you engage your inner thigh muscles, you may help to release the pull of your psoas on your lower back. However, this can take some jiggling around to get the psoas to release.
You may also find this pose slightly more comfortable if you actively press your shins into the ground. Try pressing them down while inhaling then release while exhaling.
One nice intermediate pose that can be used to exercises the abs at the same time is bent back hero. And as the name suggests you bend your back forwards so that you spine is doing a forward bend. You push your ribcage back at the same time. You can reach your arms forwards or rest them on your thighs. You may find that because this variations shifts your weight back, it's easier to get your bum to the floor.
From here you can work on gradually laying back.
For ab strengthening, do a reverse sit up and gradually lower your self to the floor. Or use your hands to support your body.
Go onto your elbows, the top of your head, then see if you can place your shoulders on the floor. Wiggle your hips if you need to. Then work at moving your knees towards each other (use your inner thigh muscles.) Reach your back ribs away from your pelvis.
Imagine lifting your head a little, and your ribcage. Pull upwards so that your abs engage. You may find that this helps to release or reduce any psoas induced pulling on the lumbar vertebrae.
Grab your elbows, or reach your arms back behind you. In either case focus on making your upper body feel long.
You can sit up using ab strength or with the help of your arms.
When doing so (either standing or sitting) I focus on activating the quadriceps.