Anti-rocker
Anti rocker is the most popular setup for aggressive skaters. An anti-rocker wheel is a small hard wheel that replaces the two middle wheels. Anti-rocker wheels almost never touch the ground except in cases where the terrain is uneven, such as skating over a ramp. Anti rockers can be made of plastic, or high density polyurethane. Some use bearings so that they can spin. Others do not use bearings so that they're lighter. The anti-rocker setup makes maneuvering harder, overall speed slower, a rougher ride, and a wider turning radii, but because of the extra space in the middle, it makes grinding objects much easier than with a flat setup.
Read more about this topic: Inline Skate Wheel Setups