The spinal erectors (or erector spinae as they are also known) are a group of muscles that run up the back of the body along either side of the vertebral column.
(In the picture you can only see them as far up as the mid-thoracic region. However, they actually extend all the way up to the base of the skull.)
They can be used to bend the lumbar, thoracic and cervical spine backwards. They can also be used against gravity or the abdominals to stabilize the vertebral column.
When active these muscles generally bend the spine backwards. To stretch them bend the spine forwards in poses like bent back hero pose or seated forward bend.
To bend the spine forwards and stretch the erector spinae, you can use the abdominals.
Note that if you have herniated disks, you'd best consult your doctor or a yoga teacher who is used to dealing with disc problems.
Stretching and strengthening the spinal erectors, the front abs and the side abs may help to avoid disc problems in the first place.
Bending the spine backwards, particularly the lumbar and thoracic spine, causes the front of the belly to lengthen and the front of the ribcage to lift and open.
You can bend your spine backwards by using the spinal erectors, located at the back of and to either side of the spine.
To use your erector spinae to bend you spine backwards, you can focus on bending your spine backwards while focusing on the area at the back of your body to either side of your spine. To make this easier you can practice bending your vertebral column backwards a section at a time.
You can focus on back bending your cervical spine (the part of the vertebral column between your ribcage and head,) your thoracic spine and then lumbar spine (between your pelvis and ribcage) individually and then all together. With practice you should then be able to both contract your erector spinae at will and feel them contracting.
To purporsefully contract any muscle you can focus on feeling the area that the belly (or bellies) of the muscle is in.
The muscle belly is the part of the muscle that does the work.
You can also focus on feeling the action that you are trying to do while using that muscle.
You can purposefully contract your spinal erectors while bending the spine backwards in a standing, sitting or bending forwards position. Better yet, if you learn to bend you spine backwards by contracting these muscles you can also learn to feel when it is bend backwards without contracting these muscles.
This can happen in a yoga posture like upwards dog by virtue of the position that your body is in.
Bending the spine backwards can cause the front of your ribcage to lift and expand. Likewise, lifting and expanding the sides of the ribcage can cause the spine to bend backwards. You can use your intercostals, the muscles located within the spaces between each set of ribs, to expand and lift the ribcage (These same muscles can also be used to "contract" the ribcage.) and cause the spine to bend backwards (slightly.)
The feeling is different from that of using your erector spinae.
Once your can feel the effects of using these two sets of muscles individually, you can choose which ones to use depending on the situation you are in... or you can use them both together.
Because the spinal erectors can be used to deform the ribcage, they can be used to assist in the act of breathing.
You can use breathing exercises as an excuse or opportunity to practice activating them and deactivating them. You can first focus on activating and relaxing your spinal erectors to repeatedly bend your lumbar spine backwards. Then you can focus on using them to practice bending your thoracic spine backwards.
In a seated position, either on a chair or on the floor, slowly bend your lumbar spine backwards as if bending around a barrel that is behind your body. While exhaling allow your lumbar spine to bend forwards.
If you have tight hip muscles or tight hamstrings you may find this action easier to do while sitting in a chair.
Because in this exercise you are trying to focus on feeling and controlling your erector spinae, focus on the movements of your spine rather than on the movements of your pelvis.
The Lumbar vertebrae are about three inches wide from side to side, so limit you awareness to an area about an inch and a half to either side of the center line of your spine. Contract this area while inhaling and bending backwards, allow it to relax and lengthen while exhaling.
When bending the thoracic spine backwards you can focus on an area about three inches to either side of the center line of the vertebral column. That is because in this area the spinal erectors connect to the ribs as well as to the vertebrae of the spine.
If you start by tilting the pelvis forwards, you can then slowly and smoothly contract the spinal erectors that act on the lumbar spine so that the lumbar spine bends backwards. Working upwards, you can continue this bend into your thoracic spine, using the spinal erectors there to bend your thoracic spine backwads.
The feeling created can be like drawing the spine forwards into the ribcage.
Note, that because of the design of the thoracic spine, it may actually just straighten instead of assuming a backwards bent shape. However, the direction you are moving in when using the spinal erectors is a backwards bending direction.
Although the erector spinae do extend through the cervical section of the vertebral column, I would suggest that while sitting or standing that you focus on pulling the head back and up while pulling the chin in towards the pit of your neck. This action tends to lenghten the back of your neck rather than bend it backwards. This action may use the spinal erectors but in combination with other muscles of the neck and shoulders.
You may notice that a side effect of this action is that it assists in lifting the front of the ribcage.
Do this while inhaling.
While exhaling you can allow the head to move forwards and down.
An exercise for purposefully activacting the erector spinae at the neck to bend the cervical spine backwards is to slowly lift and then lower your head while laying on your belly.
In this exercise, which you can use as a prelude to shalabasana or locust pose, focus on keeping your ribcage on the floor. Instead, slowly lift your head and focus on bending your cervical spine backwards. Once you've exercised and felt your spinal erectors activating at your neck you can then focus on lifting your ribcage and bending your thoracic spine backwards from top to bottom, again while using your spinal erectors.
The advantage of doing this while laying down is that you can contract your spinal erectors against the weight of your body.
With respect to both your cervical vertebrae and your thoracic vertebrae, if you lift and lower your head and ribcage slowly enough your can use this exercise to practice feeling the individual vertebrae of your spinal column as well as your individual ribs.
With enough practice (move slowly and smoothly in both directions) you should be able to both feel and control your erector spinae. You can then expand your awareness outwards to notice the side effects of using these muscles. You can notice and feel how when you bend your lumbar spine backwards you move the front of your ribcage away from the front of your pelvis, causing your belly to lengthen. You can feel and notice how when you bend your thoracic spine backwards your front and even side ribs lift and expand.
As a yoga teacher, I'm constantly exploring new exercises, new ways of doing yoga poses.
There is no single "right way" of doing a yoga pose. Instead, there are options. And the better you are at "feeling" your body, the better you can get at choosing the right option for your body as it is now.
For any technique, the point of practice is to learn feel it and to control it, so that it can be used without thinking about how to use it.
And that is more or less the approach taken in all of my ebooks and videos. They help you to feel your body and control it so that you can work towards using it effectively in anything that you do.