Back Flexion...

1 - Salabasana - passive (preparation of the groins)

Exercise for two. 
It takes 2 bolsters and a chair. 
The student lies on the two bolsters, belly directed towards the ground. The distance between the bolsters depends on the flexibility of the student. They will be brought together as long as the groins do not lose touch with the ground. The pupil places his hands on the edge of the chair - arms are stretched. 
The partner presses the student's gluteus towards the heels and outwards. This has a double effect: both the groins open more and this allows to give more space to the lumbar.

2 - Ustrasana "to the flow"

You need a chair. 
The knees are at the level of the ischium and slightly apart (width of the pelvis). The sacrum is pressed against the edge of the seat - and one leans back to get the back of the skull on the back. The arms run along the feet of the chair. 
As the pelvis and the head are supported, one can in this exercise concentrate on the opening of the "armpit chest", that is to say of the zone that is under the armpits at the height of the chest.
This is a good variation for a sequence with a restorative tendency or for beginners.

3 - Ustrasana on the wall

I do not comment: does everyone know, or else?

4 & 5 - Kapotasana "drop back" preparation

In variant 4 a chair is used to land the top of the skull on its seat, the thumbs are at the level of the sacrum, they will be lowered beyond the crease of the buttock as one leans back. The pressure of the hands in the back of the thighs helps to keep the pelvis forward and the thighs vertically as long as possible, while the opening work is done at the level of the armpit chest.

In variant 5, we are back to the wall in the starting position for Ustrasana. Here the elbows are bent at 90 ° and the hands arranged on both sides of the trunk, without contact. One leans backward keeping the pelvis above the heels as long as possible (this time without using the thumbs as in the previous variation). Then the tilted arms are tilted backward to reach the wall and eventually to descend to touch the ground. The distance between the feet and the wall must be decided according to the flexibility and the temerity of the pupil.

6 - Ustrasana in the strings

I admit it at once, I loved this variation of Ustrasana which was very light! An anti-slip mat should be placed under the shins on the wall to avoid accidents. The knees are level with the pelvis along with a horizontal line. Needless to say that the rope is at the level of the sacrum and not in the lumbar. And then we lower our arms to the ground or we catch the ankles for who can.

7 - Preparation in Kapotasana

It takes a chair and a wall. 
It is also a very good exercise. Here the knees are on the ground on a blanket - they are slightly apart and the shins completely in contact with the wall and parallel to each other. 
The pelvis is in contact with the chair and palms on the seat. One uses the support of the hands to flee towards the rear in order to bring the top of the skull against the wall. The arms are as in Bhujangasana - slightly bent. The more the chair is away from the wall the more the back bending is intense. It is necessary not only a good flexibility of the trunk but a good opening of the groin and the quadriceps to practice correctly the variation of this advanced posture.

Via: Iyengar Yoga Notes.