It's ok for your hand to be bent back as long as the BAR is over your wrist. That means you can have your hand cocked back, but the bar has to be in the lower part of your palm so that your wrist, elbow, and the bar are all on the same plane. If the bar is on the back part of your palm, like by your callouses and the base of your fingers, you're asking for trouble.
Make sense?