In reply to RyanCTJ:
A crimp is any hand position that warrants a steep angle (45' or less) on the uppermost of the two finger joints while the first stays shallow and almost straight. This refers to the position of the fingers but the type of crimp comes from how the thumb is used in conjunction with this very powerful position:
- Full crimp: Thumb extended over the top of the hold alongside the fingers, or 'rolled' over the top of the index finger as you mentioned. Basically, a full crimp is any crimp that warrants direct pulling help from the thumb and puts the hand into a closed position (creates an 'o' shape between your thumb and forefinger).
- Half crimp: Fingers fully crimped while the thumb stays to the side of the hold and offers only marginal help (creates a sharp 'c' shape between your thumb and forefinger).
- Open crimp: Very similar to the half crimp but the thumb plays no part in any pulling action and remains in the air without touching the wall.
Which of these three you use depends on the situation, but notice that you still have option to use any or all of them without having to roll your thumb over your index finger and potentially cause yourself an injury, especially while you're working your way back into the sport. I very rarely use a full crimp with a rolled thumb, it's saved for moves that require a lot of power and I tend to use it sparingly. I've occasionally popped my thumb a little over the top of the hold AND the index finger if the ledge is just big enough to warrant it, the extra power this affords is worth it if used once-in-a-while.
As for the rest, I use a combination of all three styles, usually without consciously deciding and just going with what feels best at the time; although you might find you can eeck out a little more strength by varying your grip if you're struggling on a particular hold.
In any case, I hope your return to climbing goes well and injury-free, and that you master this grip without too much fuss!