A monkey in your face! The lock attendants (hard-hat and lifejacket, in this image) throw thin ropes onto the deck, aided by a rope-wrapped weight the size of a baseball. That weighted end is called a "monkey face" and getting it in one's face is probably how it got its name. Each line handler, on the transiting boat, grab their respective monkey faced rope and attach the larger mooring line (blue rope). The attendant uses his thin line to haul in the mooring line and then loop it over a mooring cleat on the chamber wall. The line handlers on board the transiting vessel are then responsible for taking up the slack, or letting it out, as the water level of the chamber changes.
In this image the Panama canal pilot is pointing at a chamber-wall cleat (darker blue shirt), while one of the rented PanaMuscle line handlers prepares to take up the slack on his stern line. There's a lot of activity, then a gentle ride down as the chamber drains while the line handlers let out more line.⇽ BACK ⇽