The technique is called "roll-tacking". The purpose is to speed up the tack, and the roll and then rapid hike down on the new tack serve to accelerate the boat. There are several parts to thie whole move. Firstly, the first part rolling the boat on top of you actually occurs when the boat is going through head-to-wind. This causes the boom to cross the boat rapidly. The helm then crosses the boat while letting out an arm-length of main sheet. The boat turns quickly onto the new tack. Then the "pump" is when the boat is rapidly hiked flat again. As the mainsail comes rapidly upright it shoots the boat forward - think of it as pushing the air backwards as it comes up. Specifically because this technique is so effective, there is a racing rule that says you may not come out of a tack faster than going into the tack!