Basically variable valve timing, but instead of using a servo motor to advance the cam, it uses weights. When the cam spins up, the centrifugal force causes the weights to follow a path. Those weights are connected to the cam (or a planetary gear or something like that) which in turn rotates the cam relative to the cam chain/ignition/crankshaft.
Basically this, but not using an electric servo motor.
I agree, they usually do put the variable cam timing on the intake, so seeing it on the exhaust is odd - but if they are only interested in changing intake - exhaust overlap, it could work with either cam.
I don't think it's a decompressor as you would want that active at zero speed - so the centrifugal bit would be useless - unless it kicks in at 500rpm - there are easier ways to do decompression. Most modern start systems don't need decompression - Royal Enfield is an exception
They use two counterweights to keep the assembly balanced, but the decompression works on both cylinders. The system keeps one exhaust valve per cylinder cracked open during starting via a little sliding protrusion on the cam.