This question led me to this video, apparently of Dejan "STIPKE" Stipic, doing various feats of bodyweight strength. In the pull-ups, dips, and squats, there's a fellow beside him who gives a light slap to the limb in action at the top of the motion. You can see it being done with some of the other background athletes as well. Is this a standard thing to do when spotting someone in bodyweight movements? Does this do something physical like help the muscle to relax (seems unlikely to me, since they only seem to be tapping one side) or is this just a way to let the person know they've hit full extension (possibly hard to tell on multiple reps due to muscle engorgement causing numbness)?
They are acting as counters for competition. The person beside them is a judge, and the tap is to let them know that they completed a successful repetition, usually accompanied by a verbal count of the reps.
There are some types of slaps that are used in boxing and other similar training, to get the muscles used to impact and the pain sensation so that it can be ignored, but this is just a "hey, that's one" type of tap.