Seated twist is hardly the sexiest yoga pose in the world. Sit with your legs crossed and turn your ribcage to the right. (And then to the left.)
I use it a lot just because it is so simple and because it is easy to spice it up.
In this variation you can focus on activating your side gluteals and spinal erectors. Why? One reason is that it may help you to improve tone and control of your multifidus and rotatores. These multifidus can be used in the lumbar region to bend the lumbar spine backwards. Meanwhile the rotatores can be used to turn the thoracic vertebrae relative to each other.
In general this exercise will give you a good feel for activating your spinal erectors and you may just find that they help you to increase the opening in your ribcage as you twist it.
Do you want to activate these muscles everytime you twist? Not necessarily. However I would suggest developing enough control that you can activate them at will. Then you can determine for yourself, by activating and relaxing, whether you want to use them in the particular pose that you are doing.
For now the focus is on using them in a seated twist. And prior to activating them while twisting, practice activating them while bending your spine backwards.
Sitting with legs crossed, tilt your pelvis forwards and backwards.
Keep your hands together in front of your chest.
As you tilt forwards use your side gluteal muscles (at side of hip) to tilt your pelvis forwards. They may be activating automatically. Put your awareness on the sides of your hips and see if you can feel them activating as you tilt your pelvis fowards.
After activating and relaxing your gluteals then focus on bending your lower back backwards (increase the normal curvature). Concentrate on squeezing your lumbar spinal erectors as you bend backwards. Relax while allowing your spine to bend forwards.
Once you are comfortable with activating your glutes and lumbar spine erectors, then add to this and activate the spinal erectors in your thoracic spine region. Use them to bend your thorax backwards and open your chest. Relax while exhaling.
Rather than doing all of these actions together, activate them one by one, first your gluteuals, then your lumbar spinal erectors and then your thoracic spinal erectors.
Next try these same actions while twisting
With your legs crossed, turn your ribcage to the right. Maintain the twist and then focus on feeling your gluteals activate each time you tilt your pelvis forwards. Relax them as you tilt your pelvis backwards.
Then as you tilt your pelvis forwards, focus on deliberately activating and feeling your lumbar spinal erectors. Bend your lumbar spine backwards. Relax when you tilt your pelvis backwards.
Then add your thoracic spinal erectors. Bend your thoracic spine backwards along with your lumbar spine. As you relax, keep turning to the right. As you slowly bend your spine backwards and then relax work at gradually deepening the twist.
Move slowly and smoothly. Stop if you feel discomfort or pain in any part of your spine.
Rest and then do a seated twist to the other side.
Where the spinal erectors can be used to bend the spine backwards you can use your abs to bend your spine forwards. Again, practice first while facing the front and then add the same movement to your twist.
To start with, after rolling your pelvis forwards use your rear glutes (gluteus maximus) to tilt your pelvis backward. Relax while rolling forwards. As you roll your pelvis back focus on pulling your pubic bone up and inwards.
Doing the same movement activate your abdominal muscles by drawing your sternum down and back, towards your pubic bone. Pull you ribs down and in also.
Relax while bending your spine backwards.
Try the same activations while doing seated twist to the right. Relax while bending backwards. Squeeze your abs while bending forwards. For both actions, maintain the twist to the right.
Then do the same thing while doing seated twist to the left.
In this case you are using your abdominals to bend your lumbar spine and thoracic spine forwards.
In the first case you used your spinal erectors to bend your lumbar and thoracic spine backwards.
If on top of feeling your muscles activating you focus on feeling your thoracic spine as you twist, you may increase the likelihood that you are activating your rotatores.
Also, while bending your lumbar spine backwards you may be activating your multifidus muscles.