We often tend to think of bridge postures as backbends, this seems misleading causing us to focus on the arch of the back rather than on the raising of the hips, of the pelvis. In Vinyasa Krama the bridge postures are a continuation of the pelvic lifts, the dwipadapitam (table poses), of Day59, the division into two separate subroutines is arbitrary.
Ramaswami describes these postures as an arching of the torso, we achieve this by 'pressing down' the arms, neck and back of the head in Madhya sethu, by 'anchoring' the back of the head and neck in the Madhya sethu vinyasa (pic 3), 'anchoring' the buttocks and crown of head in uttana padasana, 'anchoring' the crown and feet in sethubandasna and 'pressing through' the palms and feet in Urdhwa dhanurasana.
As a rule of thumb 'a back bend begins in the toes and ends in the fingertips', the whole body is engaged rather than just a bending of the spine.
'Backbend preparation begins in postures that strengthen the ankles, legs, hips, pelvis, shoulders, arms, neck.
In standing 'backbends it's easy to fall into the mistake of immediately bending the back in the supine postures because the back is lying on the mat and can't bend backwards we are forced to consider the other factors.
In supine we tend to think more easily of arching the back,in Ramaswami's words we 'arch the torso', ground the neck and shoulders or crown of head and lift the chest, creating an arch in the spine.
We also continue the pelvic lifts of Day59 and grounding our feet lift our hips, the legs are engaged from the toes, through the heels, calves, back and front of thighs all the way up the the hips.
The tailbone is tucked in and held in place by mula bandha, creating space for the spine to avoid any compression as we arch up.
Ground/anchor the crown of the head and as we arch up by lifting the chest and hips, roll over to the forehead, be sure to drop the shoulder blades down the back so as not to hunch the shoulders and compress the neck.
The necessary Shoulder strength can be built through surynamaskaras, lead in's to the sequences, practising the arm raises of tadasana vinyasas daily as well as some of the easier arm balance postures.
Take the urdhava dhanurasana position with the hands over the shoulders and press down without lifting up, just engaging the hands , arms and shoulders, raise the hips as in dwipadapitam but don't lift the shoulders just keep them engaged and breathe, three to six long exhalations.
When your ready to lift the shoulders, raise up for one breath and lower, repeat one more time than feels comfortable, try to include a few minutes on this every practise.
Rather than thinking about the arms, think of lifting the chest, rather than thinking about your legs thin about lifting the hips.
Once lifted work on bringing the shoulders the chest further over the arms as if your being lifted and drawn backwards.
As we walk in we are creating a deeper arch or rather a higher arch, think Cathedrals, lifting the hips higher and higher.
Walk the hands towards the feet rather than the other way around.
We don't want to compress the spine but rather create more space for the arch to take place, tuck in the tailbone, engage mula bandha strongly to keep it tilted, push out the chest, drop the shoulder blades down the back so as not the compress the neck.
As you walk in breathe, settle the breath, try to elongate the exhalations to relax into where you are, inhale strongly, hold and step in, exhale slowly.
Each time you step your hands closer try to take three steady breaths, if your feel comfortable take another small step closer to your heels. When you have gone as far as feels 'comfortable, step back out and take a couple of breaths until you have come back to where you first raised up then lower back to the mat.