Same concept as the ol' "Airbox mod" in my sportbike days. It's known as the Second Air valve or P.A.I.R. valve on most bikes. It takes unburnt fuel mist from inside the air box and re-routes it through the exhaust valves to burn it off quicker. Kinda of a "smog" related thing. Block it off, and the unburnt fuel makes its way through the intake valves and eventually burns its way out, but will also crud up your intake valves a little quicker.
But the reason it decreases backfiring is because there's not liquid fuel mist making its way into the exhaust to burn off
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