Crankcase Decompression Regulator
Right under the air filter housing on the rear side.
Basically it lets combustion gases that make it in to the crankcase to be burned by dumping them into the intake.
When there is pressure from the crankcase towards the intake it is open and allows flow.
When there is a vacuum from the intake to the crankcase it closes to prevent suction, but will open again once the situation of vacuum is gone and there is pressure again.