My informant is a sailor, and has been since she was young. Sailing practically has a language of its own, and requires a full knowledge of the various parts of the boat before one can sail correctly. This is especially important in racing. My informant competes in sailing races for her university’s sailing team.
In a spinnaker boat (a spinnaker being a specific type of sail you use for downwind sailing) when the sail is up, and you need to take it down, someone, usually the bowman, yells, “Blow the guy!”
The guy is a “line” different than a “rope”, that is attached to a pole and to a corner of the spinnaker, that helps control and stabilize the boat. In racing, when a team reaches a mark and needs to take the sail down, the bowman will yell the phrase so another person can take down the line to pull in the sail.
All the racers she sails with, as well as older sailors, use the phrase. It is predominantly used while racing as there is an emphasis on haste.
“Blow” here refers to “kill,” “blow out,” “take a blow” as in a “hit” that will take the sail out. She was unsure of why the line is named a “guy.”