Control line was the first successful means to actually control a model airplane in flight. Before that, models were free flight.

There are usually two lines from the handle to the airplane. They connect to a bellcrank that translates the movement of the two lines into a push-pull movement of a connecting rod that goes to the elevator. The two control lines work very much like the pull-pull rudder setup that is popular in RC nowadays. If you pull one line, the elevator goes up. Pull the other line and the elevator goes down.

The lines are usually fine wire nowadays. It's actually pretty awesome wire. It's usually around .015" in diameter and is actually 7 strands of braided wires. That's the size you'd see controlling a .35size model. And the lines would be around 60feet long. Strong and flexible.

The airplanes can be described as going around in circles but that's a pretty weak description. Speed models have flown over 200mph. We had racing events with 4 airplanes at a time in the same circle flying races that required refueling stops done by pit crews that could refuel and restart and launch the plane back into the race in seconds. We fly a couple of models at the same time in combat events where both have crepe paper streamers yet are going way over 100mph. They try to cut each others streamer. There are Carrier models that simulate WWII aircraft carrier airplanes. And some clubs have built the carrier deck to fly off. CL scale was the first time modelers could build museum quality into a model and have a hope of getting the sucker off the ground AND back down in one piece. CL Precision Aerobatics is amazing and unbelievable. You won't believe the maneuvers. All done on 60' lines. And you will be blown away by the craftsmanship and quality of those stunters.

There actually are still a bunch of guys flying CL. The middle size town I live in has a park with two paved CL circles in it. It was built in the 60s and has had flyers using it ever since.

CL is still amazing stuff.