If it arrives at exactly the moment the valve opens to dump spent exhaust, the low pressure there will help suck out the cylinder’s contents. “If you have too much back-pressure, it’s not allowing ...

David R. Jones, Fatigue Behavior of Exhaust Valve Alloys, SAE Transactions, Vol. 89, Section 2: 800253–800756 (1980), pp. 1319-1332 ...