Here the blue boat rounds the leeward mark slightly ahead of the red boat.  The blue boat tacks.  When the boats come back together (position 3), red is on starboard.  If the blue boat tacks, they will be a position 4, and the red boat will control the blue boat.  The red boat would be able to sail to the layline, or past it, and tack for the finish ahead of the blue boat.  If the blue boat at position 3 instead dips (sails behind) the red boat and then tacks, he finds himself at position 4a.  The blue boat can then prevent the red boat from tacking and is in control.  Another option would be for the blue boat to continue on after dipping.  He would then have cleaner air, and when both boats tacked for the finish, the blue boat would have the starboard advantage.