What is the purpose of the exhaust power valve system (e.g. YPVS or KIPS) on a two-stroke motorcycle engine?

1. To act as a physical restrictor to comply with learner regulations
2. To shut off the exhaust completely during deceleration to prevent backfiring
3. To direct exhaust gases back into the crankcase to pre-heat the fuel
4. To vary the exhaust port height to optimize low-end torque and high-end horsepower