alright here we go---
Scavenging:
-exhaust gases released into the header pipe create a pressure wave
-as this pressure wave travels down the length of the header primary tube, it is generating a suction wave inthe pipe leaving a low pressure area in the combustion chamber
-as the pressure/suction wave exits the primary tube, a reflective pressure wave begins traveling back up the primary tube and can enter the combustion chamber as reversion
ok so your right scavenging and backpressure are not the same but reversion is the same thing as backpressure