http://www.velonews.com/tour2007/tech/articles/12570.0.html
That is a hollow cassette.
That is a hollow cassette.
The new cassette relies on Open Glide shifting technology, but its steel construction is the catchy story. The larger eight cogs of cassette are milled and machined out of a single solid piece of steel. The inside is milled away except for the smallest cog, which is splined. The back of the cassette has a pressed on aluminum cover, which is also splined and the production version will have a sleeve that connects the two pieces so they don't gall an aluminum hub's cassette body. This leaves the inside of the cassette completely hollow and brings the weight down to around 160-grams, according to Ritzler.
An attribute that should make it last a long time, but also keep its price in the range of Shimano Dura-Ace with a weight in the range of an all-titanium Campagnolo Record Cassette.