Any sustained muscle stretch will eventually bring about a relaxation response Yoga bolster in any muscle.5 Naturally, any painful stimuli that appear during that same time period Yoga bolster will have a contrary, unsettling, and excitatory effect. The yoga poses that have lasted over the centuries succeed in accomplishing sustained stretch and relaxation without undue antagonistic, painful, or arousing stimuli.
Speaking of antagonistic, a third reflex is also, albeit very generally, at work during yoga postures. Every skeletal muscle has an antagonist, or direct counterpart: one set of muscles clasps the hand, another opens it. In order for one group of muscles to create a bend at the elbow, the muscles that straighten it must relax. This phenomenon, the agonist-antagonist reflex, is coordinated in the central nervous system and is nearly ubiquitous in Iyengar and Anusara Yoga. If you care to stretch the hamstring muscles, proceed by tightening the quadriceps, and the hamstrings will miraculously relax.