Two drums on the yarder, The Mainline and the Haulback. The Main line is the one the carriage rides on and the haulback is attached to the carriage. So, as Frank says release the haulback line and the carriage rolls down the main line. The "tooter" is operated by one person down the hill and is called "The Whistle Punk." He signals the yarder operator when to stop or start the carriage. He can also tell the yarder operator to slacken of thighten the main line thus lowering or raising it.
I believe the motorized carriages allow the cables which attach to the logs, chokers, to be lowered or raised via radio signals so that would fall to the whistle punk to control. Also there might be some motive power to allow the carriage to go up hill on a really long main line reach that won't straighten out from just pulling on it. Main line is heavy. Some of the bigger yarders can reach out over 4,000 feet. Since the main line can be over 1" in diameter, a third drum is also on the yarder called the Haywire. The hay wire is generally a 3/8" line used to move the other lines around the area that is being logged. It's not big so it can be pulled around stumps and snags left in a clear cut for the birds. The area under the main line is referred to as a road. When the mainline is moved it's called changing roads. A logging show is where the logging is going on. If a company is logging more than one location each location is called a side. You can have more that two sides going at the same time.
One toot is stop if your moving and start if your stopped. I don't know all the signals but combinations of long and short signals are what is used. One long in the afternoon is lunch time or later in the day it's the "let's go home" signal if the rigging (the carriage) is on the landing. Standardized signals by law.
Originally of course all this was done with hand signals. When electricity first came to logging a wire was run over the bank to where the whistle punk was but out of the way of the path of the incoming logs. He pretty much stayed clear of the actually logging for safety reasons because he was the one with communication to the yarder operator
