This is a two-sided shock valve piston. One side is for compression, the other for rebound. The side shown is for compression. Notice the way that six of the holes on this particular side of the piston feature porting, which allows the shock oil to pass around the rebound shims or valve stack in a uniform fashion. This particular piston design allows the piston to flow twice as much oil in the compression direction as it could on the rebound side. Here is how it works: As the shock shaft compresses, the oil flows through the six ported holes on the visible side of the piston. When the shock shaft extends, the oil flows back through the three holes on the visible side of the piston. Pistons can be configured in several different ways, depending on the intended use of the shock. The smaller bleed hole allows a small portion of oil to flow through the piston without restriction. Sometimes a piston can have multiple bleed holes. Typically, bleed holes soften the effect of the valving.