Draining mash tun to finish sparging

What would be the difference in efficiency between these two methods of fly sparging
1. Leave two inches of water above the grain bed the entire sparge until you collect your boil volume. Do not drain the tun into the kettle. You will have leftover wort
2. Leave two inches of water above the grain bed until you use up all of your calculated sparge water. Then completely drain the tun to get boil volume

Basic chemistry would say that the latter would Get you higher efficiency, however practically I doubt you'd notice a difference that late into the sparge

There's lots of different efficiencies. Which one are you talking about?