The common DIY one consists of an angle drill with a winch socket adapter. They can be either powered by battery pack or with a long cable so it can be used around the cockpit in different winches.
Same as the commercially available ones, they are inserted after the rope is put on the winch. Then you need one hand to hold the handle against the torque generated. As this may be inconvenient, it's also possible to make a mount for the rear end of the powered handle so it can be used without needing to hold it. I expect that's what that socket is for.
Perhaps the previous owner didn't want to give up his angle drill and so it wasn't included with the boat
This one is from a mountaineering rescue system, but using a standard sailboat winch and illustrates what I mean pretty well (although they didn't do a nice stainless steel job for the holder):