A semi-humorous usage for the most important bit of a Lighting geeks' anatomy. The "Go" Finger is the digit that said Lighting Person presses the Go Button on a light board with, thereby launchning a Cue, Program, Effect, etc. The "Go" Finger is a powerful impliment in.. err, on the hands of the right person. Not all boards have a go button, per se, but it's accepted in those cases that "Go" is assumed to be whatever button is used to initiate a sequence or continue a program.
And, of course, the most supreme threat one can issue to a Lighting Tech is "I'll break off your 'go' finger and use it as a pin on the pinrail!"