Do you mean keeping your back concave in this pose - Supta Padangusthasana I ? In most of the variations of this pose in this sequence I would not be focused on trying to keep the back concave, in fact with the bottom leg bent and then when we take the bolster under the middle back the lower back would be settling toward the floor. If we practice the variation with the lower leg straight along the floor and the top leg at a right angle then the neutral lumbar curve should still be there, which may be what you are considering in relation to keeping the back concave?
