You'll probably find a lot more information on this by searching for "turgor pressure" which is essentially hydrostatic pressure.
Basically picture a fungal cell, which has a strong cell wall (a requirement for cells to experience hydrostatic pressure, or else they'd just lyse, or break open), and a bunch of water inside it. The water is not moving and thus is static, but the pressure inside the cell depends on how many water molecules are contained inside the cell wall. More water molecules within a confined space = higher hydrostatic pressure inside the cell.
The main relation to mushroom cultivation is how well fungi penetrate and colonize a substrate. A high hydrostatic pressure inside the fungal cell causes that cell to bulge and push harder against uncolonized substrate. The harder the fungal cell pushes against the substrate surface, the more penetration you get into the substrate. There are other factors as well, such as enzymes that the fungal cell uses to break down complex macromolecular structures of the substrate. But the rate of movement is directly correlated to how high the water content inside the cell is (the hydrostatic pressure inside the cell). As the hydrostatic pressure inside the fungal cell drops, so does the rate of substrate colonization. It is part of how fungi move.
-------------------- "It is dangerous to be right in matters on which the established authorities are wrong." Francois-Marie Arouet
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