When you tell your waiter/waitress to 'keep the change', preventing the need to leave money as a tip at the end of your meal. Also negates the need to calculate how much you need to tip, because it seems casually generous.
Can only be used when you pay for your food when you order it, as opposed to the classic method of leaving extra money with your bill when you've finished eating.
Waiter: That'll be £9.68 please, sir.
Guy #1: *Hands over £10* Keep the change.
Waiter: Oh, thanks very much!
Guy #2: Nice autotip, man.
Guy #1: I know, that's nowhere near the socially acceptable 10% tip! I am a genius...