The recess in the top of the direct injection piston is a swirl chamber. As the piston reaches TDC, the air is forced to move in a motion similar to the shape of the swirl chamber. This helps insure a complete air/fuel mixture so that the combustion process is as smooth and complete as possible. This also helps to equalize the pressures in the cylinder during combustion - avoiding excessive stress on internal components. Even prechamber diesels have some form of flow control on them. Look at an MB piston. The prechamber detent in the piston is nearly on center, as the prechamber protrudes on center. Combustion gases push down evenly on the top of the piston. VW IDI diesels have an off center prechamber. The top of the piston has two circular patterns connected at the bottom where the prechamber expends the burning fuel. It is meant to cause the burning fuel to swirl in the cylinder while pushing down on the top of the piston.

Now if I could only fix the damn things.