I am very familiar with it and have a swinging grill like the one below specifically designed to make it. That said, there are regional differences in what is referred to as Schwenkbraten. The one I am most familiar with are individual thick (1-2 inches thick) cuts of pork or even pork tenderloin. In the town where I had it extensively, it was referred to as Spiessbraten, but regionally it is referred to as Schwenkbraten. The other one is an actual roast.
For doing the individual cuts, I normally purchase a pork loin roast and cut it myself about 1.5-2 inches thick. Use the end pieces as the center cut is too lean. The recipe I am familiar with is very simple because it just seasons with salt and pepper and then puts the pork in a bed of strong yellow onions with onions also covering the meat sealed with plastic wrap for 6 hours or so. You want as much of the onion flavor as possible to flavor the meat.
The flavor in Schwenkbraten comes from cooking it over an open fire using beech wood (Buchenholtz). You can substitute oak or another hard wood, but it is worth using beech if you can find it. The swinging grill allows the hot flames of the open fire to slowly sear the meat almost like a reverse sear, if that makes sense. I have tried to do with on a Weber kettle using pieces of beech wood, and the flavor was close, but it missed that taste that only the open fire could give it.