Discharge chemistry can be altered. (Properties of deposited films can be altered, Etch/deposition rate can be maintained despite lower power.)
The time variation in the source power changes the electron energy and density. Consequently: Both higher and lower energy electrons can be formed to react.
When the power is off:
electron energy goes low, attachment can become large!
negative ion chemistry can be different than neutral or positive ion chemistry.
Loss rates for neutral radicals can vary significantly.