The watch that came in from the cold

"He would then set up the poles, string the line between them, and attach the harness. The C-47 would then come in low and slow, trailing a grappling hook. The hook would snag the line, lifting the harnessed operative off the ground and into the air behind the airplane, where he’d be winched on board."

And here's a written description with some photos. The photos include one of a C-47 about to do a pick up: https://www.ww2gp.org/gliderpickup/