Although we're holding on to the big toe and these are often referred to as big toe stretching postures it's the complete other end of the leg we're focused on,where the head of the femur enters the pelvis.
Remember this is a ball and socket joint, rather than wrenching the leg back there is a gentle drawing back of the leg that allows for some rotation in the joint in akrarnadhanurasana.
In Kraunchasana as well there is a slow raising of the leg which allows for the rotation in the joint.
Although we're focussing on the hip joint here Kraunchasana gives an extreme hamstring stretch so we need to be sufficiently prepared, possibly through earlier standing postures in out practice.
NB: These are practice notes that will be tidied up and put into the new edition of my Vinyasa Yoga Practice Book along with the current sequences and subroutines. The book can be freely downloadedHERE. There is a page on Facebook HERE with all the latest sheets and updates. This book is in no way a substitute for Ramaswami's Complete Book of Vinyasa Yoga.