Two key ideas in side bending are to lengthen the side you want to lengthen and firm the opposite side. If you can learn to move from the navel upwards your posture will more effectively move blood and energy through the spine.
You can see the influence of the Yoga Synergy team of Simon and Bianca here so please also go straight to the source and see what they have to say.
What I wanted to show in this video is how to do a side-bend first in a standing balance posture and then apply it to a more traditional side bending posture like parsvakonasana.
Apologies upfront for the annoying autofocus on the camera, which leaves me in soft-focus a few times. Must get that sorted.
What you will find is that a side bending posture can lengthen and traction one side of the body/spine, while firming the opposite side. The firmness does not come by itself and you need to move the body in particular ways to achieve this.
Start from standing, feet comfortably apart, weight forward in the toes, knees slightly bent, sitting bones moving down and forward, top of the pelvis back. Relax and breathe.
LENGTHEN ONE SIDE
From the standing position above, move right toe tip forward. Move sitting bones forward. Then, to lengthen one side--let's say the left--initiate the movement from the base of the spine up. So move the navel to the left, then the ribs, then the chest, then the collar bones. If you can move the individual vertebra between each of those points then do!
Then enhance the lengthening by using the shoulder/arm--taking the left arm forward, up, and to the right. Do not worry about taking the arm too high--this can tend to squash the neck. Try to go for more forward and across movement. Play with it a little and see what gives you length without squashing.
FIRM THE OPPOSITE SIDE
With left side long, now firm the right side. To do this move the right armpit down towards the right hip. Then push the right hip forward and up. The forward and then the up are important. You need to do both. You should find this firms the right side without squashing it.
Relax and breathe into the abdomen which is firm on the right side but soft on the left. Put your neck in a comfortable position so that it is not squashed on any side.
The basis for side bending is to move from the navel up. Lengthen one side of the body and firm the other side without squashing. If you let the sitting bones move back you will end up squashing the lower back so watch for that.
You can use these basic principles for any of the side-bending postures. I demonstrate this in parsvakonasana in the video. But trikonasana would be another example of this. There are many floor sitting side bending postures where you apply the same principles of lengthen then firm as well.
Remember, don't do anything that doesn't feel good. Look for a feeling of freedom and ease rather than stretch and tension.
Happy and safe practicing!