Here's what Sentient says:

The ultimate space saver for storing your cargo bike in a hallway, stashed in your van, or next to your bed!

Combine this Twist stem with a Tilting Rack Bracket on your Omnium Cargo or Mini-Max and instantly convert your bike from a 3d object to a 2d object.

Here's what BySchulz have to say:

Drawn in an attractive classic design, the Stem Twist SDS pleases the observer – no wonder that it won a Eurobike Award 2016 in gold. Typically by,schulz, the Stem Twist SDS is characterised by its special twist function: Just open the quick-release lever and press the release button – and the 1 ⅛” Ahead stem, forged for top stability, can be turned 90° to the left or right within seconds.

Et voilà: The perfect, tool-free solution if you want to quickly turn your bike into a “space-saving wonder”, e.g. for transport in the train’s bicycle compartment or when parking it in a cramped hallway. The useful value of the Stem Twist for (e)-city and trekking bikers is increased by its SDS front cap, which can be used to easily dock a navigation system or a bicycle lamp. If you want, you can combine two accessories.