http://www.teamassociated.com/news/l..._Shock_Pistons!
New Factory Team machined pistons for the 12mm big bore shocks take performance to the next level.
Precision CNC machined pistons allow for the best fitment and concentricity with the shock body to improve handling and consistency. The new pistons are also tapered for independent tuning of compression and rebound.

Terminology

Compression movement: Shock piston traveling upward toward the top of the shock
Rebound movement: Shock piston traveling downward toward the bottom of the shock
Tapered piston: One side of the piston face is flat; the other is tapered / angled
Flat piston: Top and bottom of the piston face are flat
Difference between tapered and flat pistons

A flat piston will travel at the same speed in compression as it does in rebound (given equal force is provided in both directions)
A tapered piston will travel faster in the direction of the taper/angle.
For example, a tapered piston with the taper facing down will travel faster in rebound than it will in compression, allowing the user to tune the rebound and compression movement independently.

#91346 FT Tapered Piston, 2 x 1.5mm holes
#91347 FT Tapered Piston, 2 x 1.6mm holes
#91348 FT Tapered Piston, 2 x 1.7mm holes
#91349 FT Tapered Piston, 3 x 1.4mm holes
#91350 FT Tapered Piston, 4 x 1.3mm holes
#91308 Standard Flat pistons, two-hole (1.5mm, 1.6mm, 1.7mm)