The system sprays water into the combustion chamber in order to lower the temperature of the air and increase the engine's efficiency, thus allowing it to produce more power without overheating, and relying less on ambient temperatures. It does require refilling a 1.3-gallon water tank in the trunk, though.
According to Car and Driver, it's that same system that BMW will be installing in the new M4 GTS, taking the place of previous M3 GTS specials. If precedent is anything to go by, the M4 GTS will also be stripped of excess weight and feature a tighter suspension, stickier tires and more, in order to make BMW's muscle coupe even more hardcore and focused for the track. We could be looking at a debut as early as Pebble Beach in August.