how do frats dirty rush?

I don't think dirty rushing is a thing for frats, like they "dirty rush" but it's just normal for them so it's not really "dirty" it's just part of their rush process

Since there isn't a formal rush like sororities have, must bidding occurs during the summer months without IFC supervision everything is fair game. Will a fraternity bad mouth another fraternity? Hell yes. Will a fraternity hosts private parties with alcohol? Yes. Dirty rushing is really a sorority thing, since they have a very structured rush, with many rules about contact with a PNM, they are being watched like a hawk by rho chis to insure they are not secretly telling PNMs anything on their desire to bid them.

What sororities would call dirty rushing is just rushing for the frats.

There is no such thing as formal contact periods, no contact periods, “rounds”, etc.