Here's a video from the Washington Post that demonstrates how the front of house staff at BLT Steak in DC communicates using secret, silent hand signals while in the dining room. General Manager Adam Sanders explains how hand signals allow the front of house staff to work more efficiently, shaving time off things like getting water to the tables and finding help clearing plates.
Critic Tom Sietsema dives deeper into the practice of restaurant hand signals, tracing it back as early as the '40s at the Stork Club in New York City and finding it alive and well in restaurant across the country including Chez Panisse, the Inn at Little Washington, and Eleven Madison Park. (EMP even has a signal for sweeping away crumbs.) To see how it goes down at BLT Steak, watch the video below and note how the service captain and the team of servers and bussers communicate silently to make sure things run smoothly on the floor: