In reply to Madden:
I've had it after I knacked my fingers doing *way* too much climbing, ignoring the pain I was getting from my tendons, and generally overdoing it when large parts of my body were telling me to "for christ's sake, STOP".
Basically my understanding is the tendon is inflamed so swells up, and then is too big to fit into the sheath. You can force it by using increased muscular pull to get it to fit in, but then it generally "pops" in all in one go, so to speak.
I ended up resting from climbing for nearly six months, and strapping up the offending digit overnight (in fully extended position), plus taking lots of ibuprofen. It went away after a few weeks, but it was, as I say, months before I could climb comfortably on that hand again.