Simply put, you only need to “break the plane” of the finishing line in order to finish. If we look back at the definition of finish, it tells us “A boat finishes when, after starting, any part of her hull crosses the finishing line from the course side.“
But, why then, don’t you have to cross completely? There are situations where doing so may not be desirable. For example, the finish line may be very near to an obstruction – a headland, a commercial shipping channel, etc. It may be that there is a strong current pushing you down-course. You don’t want to risk drifting back into the mark and touching it after finishing. It may be better to break the plane, drift back below the line and bear off. I am aware of one situation in particular where racing on the Niagara River in Buffalo, the finish line is sometimes downstream. Going any farther north than necessary is not advisable – there is a rather precipitous drop ahead! (and a sailboat is not a barrel)