As long as the knees are bent there isnt a problem. About the chin, it should not be down more than 15 degrees, where it should be is back. Why 2 reasons both with the same reasons. 1 stops the diaphragm from working by up to 50% chin back opens the airways, watch an opera singer. Plus it gives good posture, the heads in the place it should be, so you are not wasting energy by the shoulders supporting the head.