Manual seat height adjustment:
The seat is raised by turning a knob under the back of the seat.
Pneumatic seat height adjustment with tilt lock:
The manual seat adjustment knob remains for aesthetic purposes, but a lever is added to the side of the chair's chassis to allow you to raise and lower the chair at will. Tilt lock is added to help you define how far you want the chair to lean back, or keep it from leaning back at all.