The most recent rock and ice Ask Dr. J column describes this injury:
"the pain might be arising from a tear at the insertion of one of a group of small muscles in your palm called lumbricals. Unlike other muscles they attach to corresponding tendons that control flexion and extension of the fingers (rather than two bones). THese are subject to severe elongation forces when adjacent fingers are loaded at opposing lengths."
Here's another post detailing something similar:
http://www.rockandice.com/forum/viewtop ... 055f3334e0
The magazine article said the best advice is not to split your fingers on pockets (don't curl your pinky into your palm on a two finger hold). That can be hard to do, but it does work.