Air-scrawl

"All right," Web said, gesturing to the waitress for the check with the universal air-scrawl. "Thank you."

What would that gesture be?

pretend you have a pen in your hand and scribble a signature in the air.

Oh, OK. The 'Thank you' is what threw me off. For some reason, I thought the air-scrawl was referring to giving thanks to the waitress.
Thnx