What does "rap" mean for brownies recipe?
The oven shelf is not the back of the oven. Basically what you would do is lift one side a little (maybe an inch or so) and let it drop. Then you can do the other side. This will give you a more fudgy and less cake-like crumb, as the cake-like finish comes from trapped air bubbles. If you rap the pan to help the air escape then you have less trapped air and thus a denser, more fudge-like crumb.