Modern replacement for the traditional chain whip
The tool pins engage the smallest cassette cog and hold it in place, while the opening in the middle allows easy access to the cassette lock ring.
The version for 11 and 12 tooth sprockets has longer spindles on the “12 tooth” side. This makes it fully compatible with cassettes where the two smallest cogs have 10 and 12 teeth. To remove the sprocket, where the smallest sprocket has 10t, the tool goes over and engages the next 12t sprocket.