There is a train of thought in some yoga circles that using props in this pose must adhere to certain strict guidelines. Placing a block between the knees provides a different kinesthetic stimulation to the body.
Tag: Backbend
Lean Back . . . Lean Back!!
Backbends can be restoratively releasing to the front of the body and engagingly stimulating to the back of the body
Ab-Sutra Wellness and Fitness 