Oil Trap by name, oil trap by nature, it traps the soilds/liquids in the oil and allows just the hot air and vapor to breath into the airbox.

Engines need to be vented to allow gasses that build up to escape, but generaly with that gas you get some fluids/solids.

An alternative to a oil trap is a breather box, which is what I want to fit to my turbo, you basicly build a small box and fill it with steel wool or something and allow the oil breather to vent into that, it fills up sometimes with a little oil so you may need to drain it sometimes, but it's a better idea than letting oil vapor and gasses get sucked back through your engine.

Probally more info than you were after, but I was bored...