Gen rushing is the strategy of prioritising generator progress above everything else, finishing the objective before the killer can build meaningful pressure. It is less a specific tactic than the natural result of a team that never wastes time.
Its effectiveness is the main reason killers are measured on how quickly they can convert chases into hooks: a killer who spends too long on any one survivor simply loses to the clock.
In DBD Arena's ranked 4v1, generators left standing feed directly into the result for the strongest killers - an S tier killer must secure all four kills AND leave at least one generator unfinished, precisely because beating a competent team on the clock is the real test.