Drysuits create an artificial air space, much like the one created by your mask and your BCD. And just like those airspaces, the air inside your drysuit is subject to the same rules: as you descent, the unequalized air inside your suit will compress in line with the surrounding pressure. As you ascent, any air inside your drysuit will expand as the surrounding pressure drops.
You can think of your drysuit as somewhat like a secondary BCD, to keep your buoyancy in check, you will need to add air to your suit as you descend. The amount of air needed is dependent on factors such as the amount of weight your using, the type of dry suit you’re wearing, and the salinity of the water.
On descents, you will always need to add air to your suit to avoid squeezes, unless you’re making very shallow dives. When you ascend, you need to release air from the suit to make sure that you do not ascent too fast.
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