Bhujangasana is similar to Urdhva Mukha Svanasana ( Upward facing dog ) except that pelvis remains groundedon the mat.
The postures are repeated again and again in each vinyasa (variation) raising and arching the spine on the inhalation returning on the exhalation.
because they are repeated so many times we can gently, gradually, deepen the stretch as the subroutine progresses.
Raja kapotasana is an advanced back bending posture where we aim to stay for three to six breaths with the head on the soles of the feet.
In Raja kapotasana Ramaswami instructs us to keep the thighs together, this is particularly challenging.
Picture 3 is a gentler version of Raja kapotasana, a working towards it.
To work on develop our flexibility we might take a tip from Urdhva Mukha Svanasana ( Upward facing dog )
To go deeper into Urdhva Mukha Svanasana ( Upward facing dog ) where our pelvis is off the mat we are able to arch the back as much as possible but then work from the pelvis and bring it down and almost through the arms which deepens the backbend.
This is possible in bhjangasana also. Arch the back on the inhalation and at the end of the inhalation lift up off the mat enough to bring the pelvis forward a little and back down on to the mat, deeper into the posture.
In arching the back we stretch and lift out of the pelvis just as we have done in the standing bacbending hand/arm variations in tadasana.
There is also a stretch in the opposite direction, try to stretch your legs from your buttocks to your toes, this is especially important in Raja kapotasana.
The main challenge and 'trick' to Raja kapotasana is to keep the pelvis on the mat as much as possible.
To bring the feet closer to the head the stretch from your thighs rather than just the calves.
Raja kapotasana will work the hamstrings and calf muscles and we must be careful not to overstretch them in reaching for the head.