This makes cleaning a grimy, dirty cassette much easier. It is shaped nicely to rotate the cassette on the wheel while keeping the brush still, or doing it the other way around. Whatever your preference this brush does a great job cleaning it up!
Absolutely perfect for getting all the grime out of your cassette and derailleur rollers! Spray with your soap and water solution or bike cleaning product of choice and spin the pedals. The angle on the brush works it's magic and really cleans well. The bristles are stiff an durable. I use a bio based cleaner with this brush that's a little more than just soap but it cleans very well and then easily rinses out. I wish I had this product a while a long time ago. This is FAR better than a tooth brush or other softer brush you might have laying around.
I've got the other Park Tool for cassette cleaning (narrow brush on one end, plastic teeth on the other) and I'd compare two by saying this brush is the toothbrush and the other is the floss. This brush is great for getting a lot of gunk/dirt off the cassette quickly. I've been waxing my chain so I don't get a lot of oily build up on the cassette any longer, but I can accumulate a lot of dust/mud gravel riding. This brush quickly removes a lot of that gunk where it matters, i.e., where the cassette actually touches the chain. The deeper area between the cogs can wait for when I'm doing a major mid-season cleaning.