Before beginning the asana practice today, I read a passage from Beryl Bender Birch's "Boomer Yoga" (p. 82) that talks about the position of the pelvis in forward bends.   It explains what happens when someone with tight hamstrings or tight lower back rounds their back and strains in order to "look" like they are folding forward - even though their pelvis is actually tipped back.  And it reminds us how important it is to engage the quadriceps  to signal the hamstrings that it is OK to relax and let go.