Basically it is about keeping the Hull horizontal and increasing the speed by a few knots.
This is an excerpt from a technical journal on the Schnellboot!
Experimentation with S-1 and the intial batch of five additional boats led to immediate improvements and innovations. For example, beginning with S-7 (1933) the increased reserve buoyancy of a knuckle added at the bow prevented the boat from nosing into waves in foul weather. Another key innovation was the addition of a special rudder arrangement beginning with S-2 (1932). Port and starboard of the main rudder were two smaller rudders which could be angled outboard to 30 degrees. Known as the "Luerssen Effekt," at high speed, the angled rudders drew a ventilation air pocket slightly behind the three propellors, increasing their effiency, reducing the stern wave and keeping the boat at a nearly horizontal attitude. A wedge was added to the lower stern beginning with S-18 (1938). This deflected the water flow slightly downwards, counteracting any tendency for the hull to settle into the water as speed increased.