It keeps the exhaust valve from completely closing below about 800 RPM. Exactly how, I don't know, but you can see the rod coming up into the cam lobe from the weight, which must be how it does it.

I read here, about 2-3 years ago, that the cam holds the valve up off the seat just enough that it lowers the compression enough to make starting easier, and after the engine is running, that small bit of gap between valve and seat doesn't harm the running of the engine.

The way it works on larger engines is that the cam has a second lobe. The smaller lobe opens the exhaust valve for a short period of time during the compression stroke but then allows the valve to fully close again.

But on the cam above, no part of this mechanism touches anything at all. It's nowhere near the exhaust valve lifter.

If you look closely at the pivot end of the weight, you will see a little tab that bumps the cam follower (tappet) slightly to releive a little compression. Remember we are only talking a few thousandsth of an inch for an instant. Typically B&S engines relieve compression via the INTAKE valve except the Vanguard V Twins which do relieve compression via the exhaust valve. As engine speed picks up, the weight retracts the little tab.

Walt Conner