To me proper course means - the shortest course between the marks. You cannot carry another competitor way off course. That is not good sportsmanship. Just like room at the marks, you are entitled to do a tactical rounding, as long as its done in a seamanship manner. You are not allowed to go way off course.

You over stood the line, you are sailing a lower course, anything higher than required and you are no longer maintaining a proper course.
Its even worst if you are pushing a windward boat above the mark where he will miss it in order to avoid a collision with your boat.