R100GS Flip-A-Lever Throttle Lock
BMW has a complete line of accessories for motorcycles destined for police use, and one of the handier of these is a friction throttle lock. The idea is simple: It screws into the throttle grip housing and rubs on the flange of the grip to resist the pull from the carburetor butterfly valve return springs. Screw it in tight enough and you can take your hand off the grip and the bike will keep running at the same speed. Back it off a bit and the weight of your hand will have the same effect, and it will be easier to change throttle openings to compensate for hills and such. Most useful on long trips to let your hand take a break once in a while, it's also nice to be able to take your hand off the throttle to adjust your face shield, open a vent in your jacket, or wave to someone at the side of the road.
This little doodad has been around for years, for most of BMW's motorcycle line (the K1200 series is a notable exception).