Snooping around the net, I found this comment (from a user on a forum - I added the bolds):
"One thing that I have found that causes cylinder lock up is back traveling a double action trigger pull. Once you begin a double action trigger pull you can stop, and freeze at any point along the cycle, continue to increase pull and stop again and again but never go backwards or let the the trigger back travel once you have started a pull. You must complete the cycle or you can and will experience a frozen or misaligned lockwork of the revolver. I have learned this the hard way particulerly with S&W revolvers."
And mine is a S&W