This is truly a quality tool I've needed for many years. This brush & gear teeth are perfect for cleaning my rear cassette, derailer & idler wheels. Beats heck out of the toothbrush & Q-tips I've used till I got this tool. It doesn't leave fuzzy cotton between gears like the Q-tips did.
I like this because it allows me to do a very thorough cleaning of bicycle cogs. Even with regular chain cleaning, winter riding can gunk up the cogs and rear derailleurs, so this tool is great for getting to the hard-to-reach places.
Bristles are stiff and long enough to get out caked on grease on both cassette and between the small and large chain rings.
Not sure if the toothed semi-circle thingy is all that useful.