Winch launches are a lot like a kid with a kite. Run fast (wind in the winch) to get speed and up it goes. At its desired altitude, the glider disconnects and goes on its way. https://youtu.be/YePIJKs5me0 gives you an idea.

On a day with a decent breeze, it is possible to "kite" a glider up to very high altitude by letting the wire out again. But this is highly frowned upon because it means there is a very long (and nearly invisible) wire dangling right through the airspace used by other aircraft!

Thanks, I tried to look exactly this up, but couldn't find anything. Looking at that video, the only thing I could think of though was that cable, presumably detaching shortly after the video ended, and the cameraman is just standing there where the cable could presumably fall. It does not look very thick and heavy, but still!

Traditionally, winch cables are single strand steel (fencing) wire - I guess because that's cheap. But they break quite often.

Cable falls can be a problem. A cable falling over power lines causes all sorts of fuss!

Cable breaks can be terrible. The cable will spring back, whip around, and is incredibly destructive - hence the cage around the winch operator. I would not have been standing anywhere nearby and certainly not next-to or behind the drum.

The winch launches the glider and gives it speed from the ground that it can then convert into enough altitude (ie time) for the pilot to find a rising air current (ideally, I guess).

Edit: so yes, your first thought is correct. :) It's somewhat similar to an aircraft carrier launch except that the glider can get a lot more altitude out of it.

You nailed it: the winch pulls the glider rapidly up to altitude, lets go, and the glider floats back down to earth.

Not sure why you are being downvoted, but thanks for the response nonetheless.

Hah, it wasn't a very meaningful response. It's OK for this community to have moderated my comment appropriately, especially given drpixie's much better answer! Yes, it's just like a kite!