This cupped sleeved washer adds a small edge that creates extra support for the bushing. The centre sleeve and the outside cup are designed to stop the bushing deforming under stress (basically you going hot and fast through a corner). Other conditions can be a simple as tighting carving or fast pumping.
Using this cupped washer, you'll notice the turn is restricted. The preloading of the sleeve and cup immediately makes the bushing solid and the return to centre straight. Downhill with cupped washers and you'll be bouncing out of corners and setting up for the next one.
If your bushings are already hard, adding cupped washers can make it unstable, always best to experiment with your set up to fine tune and get it right.
**Here are a few generic suggestions:
Freeriding: flat washers all round = boardside, roadside - front and back
Fast freeride: Cupped washer on back boardside and flat washers on the rest
Downhill symetrical: Flat washer roadside and cupped washer boardside front and back
Downhill directional: Back truck two cupped washers, front truck two flat washers