My swim coach, an accomplished competitive swimmer, wants me (not an accomplished competitive swimmer) to curl arm at the beginning of the catch. To the contrary, Terry Laughlin of Total Immersion specifically says not to do this - that it works for pros but not a good idea for mere mortals. See 7:35 minute mark of this TI video.
Do you curl? And would you recommend curling for the swim-challenged - versus flapped wrist as Terry recommends?