This strange game is an invention of Will's - he's been doing it, and talking about "paying pockr pockrrr" or various other similar pronunciations for a month or so. The game is played by holding either socks or shoes or slippers in your hands and shaking them around to do a dance like in the video below. He often decides to play when we are trying to get him to put on the shoes/socks, delaying the process significantly.
For awhile, Erin and I thought he was talking about playing ping pong, and mimicking the moves of swinging at a ping pong ball. This would have made sense because the rec center where he and Erin go swimming on fridays has a ping pong table right outside the locker room and he is very interested in watching people play on it. But no, he can clearly say 'ping pong' - and we also talk about playing that fairly often - so pockr pockr is something different. We asked his teachers at school and nobody seems to know how he came up with this game, so the game and it's name apparently originated entirely from Will's imagination.