When you break it down, a shock is comprised of essentially two parts – the spring and damper. It’s obvious what the spring does, but what about that damper? This damper controls the internal flow of oil. In the mountain bike world, this is accomplished by either fixed or variable holes (aka apertures).
Fixed apertures can sometimes be adjusted via mechanical means to increase/reduce flow, but they’re relatively simple. Things get more interesting when we move to variable apertures. These ports dynamically adjust based on pressure differences. You’ll often hear them referred to as Boost valves, pop-up valves, or shim stacks.
It’s honestly not the easiest topic to understand, but Vorsprung Suspension has does an excellent job of breaking things down in this video.