Shimano XTR 9 Speed Cassette.
The XTR 9 Speed Cassette is a lightweight three piece aluminium carrier with 4 high performance titanium cogs combined with 5 quality aluminium cogs and weighs in at just 224g to make it Shimano's lightest 9 speed MTB cassette.
Hyperglide
In the late 1980's Shimano introduced Hyperglide which offered a new sprocket design which allows the chain to engage two adjacent sprockets simultaneously which allows the chain to mesh with the sprocket before it disengages from the old one which results in a smoother, quieter and faster shifting experience. To able this system to work the rotational position of each sprocket tooth must be aligned to that of the adjacent sprocket.