Comms between ground staff and flight crew. It is usually SOP for pilots on the 737 series to obtain a verbal clearance from the duty ground engineer before turning on the electrical hydraulic pumps (powering up the hydraulic system). This is to ensure other personnel are clear of the flaps and leading edge devices in case they are inadvertently actuated by the pilots before engine start.
In Virgin Australia airline, ground engineers who for some reason may not have intercom directly to the flight crew have a unique signalling system to tell the pilot it is OK to pressurise the hydraulics. They first rap on the side of the fuselage to get the pilot's attention. When the pilot opens a cockpit window to see what is going on, the engineer will look up at the pilot and press two fingers on his own (the engineer's) eyes and give a thumbs up.
That translates as "clear to (wait for it) Press Your Eyes....Pressurise." Neat but not gaudy like an elephants arse tied up with a bicycle chain, as the old saying goes
