 The actuation mechanism is simply a piston and cylinder, similar to a small pneumatic ram.The "tappet bore" where a hydraulic tappet would normally reside, becomes the cylinder, and the retainer assembly becomes the piston.Pressurized air (nitrogen) is pumped into this cylinder which then causes the piston/retainer to rise to the top of cylinder, causing the valve to form an airtight seal with the seat.