To prevent more confusion, let me define "back pressure".
This is when the volume of exhaust gas from the engine is significantly larger than the outlet pipe can flow with minimal resistance, resulting in a pressure rise in the pipe. If it's high enough, the engine will run more slowly until it stops (potato in the tailpipe).
When the engine speed drops to idle, the exhaust pipe is many, many times larger than necessary to permit the exhaust to exit. The internal pressure may be very close to atmospheric.
