Hes not shorting the board.
When you press a membrane button it connects two wires together.
He is just doing the same thing cept the membrane pad is broken so he has to use an allan key.
There are 3 pins going to the membrane pad.
You just need to short either of the other two. (using trial and error)
You cant break the board by shorting the two wrong membrane pins, it just wont do anything.