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What you see at the bottom is a gas separator piston which keeps the shocks fluid separated from the high-pressure Nitrogen gas that reduces cavitation and foaming in the shock that can cause inconsistent damping. Pressurization reduces cavitation and foaming by raising the boiling point of the shock's fluid, just like how a pressure cooker raises the boiling point of water to speed cooking.
There must be some gas volume in every damper to make room for the fluid displaced by the shock shaft as the shock compresses. If there wasn't any gas, and if the shock was filled to the brim with fluid, the shock would just be hydraulically locked and not move!
The holes in the Feal piston are shaped to facilitate a digressive damping curve. A digressive curve means that the shock can have a lot of low piston speed damping force yet blow off at higher speeds. A digressive curve gives a lot of body motion control in roll and pitch like a stiff shock but can blow off at higher velocities giving a good ride and good traction.
