A pre-chargeback is the notice that arrives when the buyer has already opened a dispute, but it hasn't yet been processed as a chargeback. There's a short window to act.
Refunding within that window closes the case before it counts. The amount goes back to the buyer either way, but without a fee and, most importantly, without counting toward your chargeback rate.
What makes this mechanism work is speed. An alert that sits for two days in an inbox turns into a chargeback all the same.