The term "Bottoming out" is commonly used to describe when a key stroke is pressed all the way down where the stem of the switch hits the inside of the switch casing. This causes a "clack" from the collision on mechanical keyboards. In this case, your pushbutton. Switch sound dampeners are soft rubber o-rings that can be installed onto every single button stem to cushion the landing of the switch by stopping the key stroke just short of the point of bottoming out. This also very slightly reduces the max travel distance of the switch.

This modification is popular among mechanical keyboard owners (and fightstick owners!) from around the world. Since most people will bottom out at least a fraction of their button strokes, the switch sound dampeners will cut away extra noise giving your fightstick a consistent sound output while gaming. The added shock absorption can also help with finger fatigue during long gaming sessions.

Made of industrial grade rubber which has much better resistance to cracking, tearing, and chemicals than nitrile or silicon rubber. Fits firmly onto the stem of your Cherry switch keystem. 

Black Cherry MX Rubber O-Ring Switch Dampeners 70A Hardness