So if you say the protocol is to have more intake lift at TDC than exhaust lift, then the intake valve is open more at TDC. And so the intake valve opens before TDC as the piston is moving up pushing exhaust gas. At some point before TDC the intake and exhaust valves are open equally (unless its a single pattern cam and not advanced). So there is equal opportunity for exh gasses to be pushed into the intake or out the exhaust (actually since the int valve is bigger and flows more, its more likely that exh gas flows into the intake). This of course diminishes as scavenging takes over.
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