It is done with the four front knuckles in the leopard strike position.  The leopard strike is the main way that we punch in the branch of choy lay fut that i practice.  You will get deeper penetration with it and it also gives you a inch longer on your reach for feints and strikes.  If you can pick up the inside kung fu issue with the dragonball evolution on the cover.  There is a article in it written by the grand master of my style Tat Mao Wong about the yum tsop.