Mic pumping is the squeezing and releasing of a foam-covered mic, producing a deep, muffled pressure sound. These channels make it a regular part of their trigger mix.
Mic pumping channels use the trigger as part of a wider set of close-mic sounds, such as mic scratching, brushing and tapping on the foam. Many of these creators favour no-talking videos, since the sound works best without a voice on top.
Most use a mic with thick foam covers, often a binaural mic with foam over both ears, so the sound alternates between the left and right side. Some leave long gaps between pumps and others build them into a fast rhythm. Much of the sound sits in the low end, so it comes through more fully on headphones than on phone speakers.