To barre a partial set of strings (like 2220 for example) you dont need to bend anything backward.
Just the opposite in fact, my second knuckle is nearly at 90 degrees, and then I angle the finger up slightly until that first string can ring free.
I suppose the first knuckle is bent slightly back, but not by much. Partial barres are difficult for most humans though - No argument there. I hate the damn things.
Also remember that most of the partial barre D/E shapes can be ignored by barring all four strings (much more comfortable) and dropping your pinky three frets up on the first srting (C shape). So D 2220 becomes D 2225, and E 4442 becomes E 4447. Much easier to play and no need for partial barres. It also puts your hand in a better position to transition to 7 chords, 4 chords, or just hammer-on fills since your ring finger is now free to do as it pleases.