That's part of the crankcase ventilation system or PCV. The engine sucks air through the engine to draw out oil vapors and such and burns them inside of the engine. Excessive oil coming from the PCV system is generally a result of worn piston rings. When put under load gasses leak past the rings and carry oil out of the crank case and deposit it all over your engine.

Replace the PCV valve before you get to worried about worn rings though.