The short version is that I have no idea what happened. I was on a pretty tame section of a mountain bike trail I ride a lot (this is my favorite local trail system). I must have hit something; the only thing I saw afterwards was a 2" thick stick that was about 2' long. I guess I could have come at it on a strange angle. I felt myself catapult forward and landed on my left side. I have more bruises than I can count from foot to hip and across my stomach plus my calf has a lot of abrasions. I'll probably go through my entire tube of arnica gel this week. I think my hand must have jammed my handlebars somehow when my forward momentum was stopped. It took me a few minutes to get up b/c I couldn't move the hand.
My cycling team is sponsored by a major sports medicine group in Atlanta, so we can get in quickly--holiday weekend not withstanding. The xrays were clear; no breaks! The doctor said the hand and fingers are sprained and that one of the small bones or maybe 2 have a bone bruise. That explains the pain I've been feeling near the underside of my fingers & a joint. In other words "I done whacked my hand real good." The treatment is don't do anything stupid for a few weeks, ice, ibuprofen if needed, and a couple of exercises she gave me to keep it from getting stiff. If it's not starting to feel better in a week follow up again.