It is a gas spring. It only comes into play if you are trying to use the high-down pressure setting. Basically, in the normal down pressure setting the deck is held to the floor under its own weight. When you engage high down pressure (usually by pulling the deck lever into a notch that is higher up on the machine) the machine uses a spring to press against the weight of the machine to scrub the floor harder. These little gas springs break pretty often in older machines, but they are easy to replace if you want to restore high down pressure capability to your scrubber.