The process of self satisfaction via masterbating (hand), and the eventual release of seminal fluid (fire) as a result of the act. Note that if "the fire" is not made, the term handfire is not applicable. You would be attemting to make some handfire, but would have not completed the act in the true sense of the term.
"I sprayed her down with a little handfire"
by JordanSPE March 30, 2006