Cassettes are held on their freehub body with a threaded lockring that has grooves in its inner diameter. To remove the ring you remove the qr skewer, insert the guide pin of the tool into the hollow axle until the splines on the tool mate in the groves in the lockring. Then hold any cog with a chain whip while you unscrew the tool using a wrench or socket on the hex flats. When the lockring is off, the cogs slide off the freehub body.