The rules that apply to a downwind finish line are identical to those at a leeward gate. The rules for mark room apply within the 3 length zones. Whether or not a leeward boat can push a windward boat above the finish mark depends on how the relationship between the boats was established when they reach the zone as well as how things are established before they reach the zone. Within the zone, the inside overlapped boat is entitled to room, so you have to let them in. However, before the zone is reached, the windward boat must keep clear of the leeward boat, so the leeward boat may luff the windward boat up within the limitations of Rule 17 (same tack, proper course). So if they attempt to overtake you to windward before either of you reach the zone, you can luff them. Once you hit the zone, if they're inside overlapped, you have to let them in. It's all the same stuff as at a leeward gate.

sm