There is water in three places in your body: in your blood, in your cells, and between your cells. There is an equilibrium between these three, and one of the large determinants of where that equilibrium sits is physical (“hydrostatic”) pressure. When you compress the tissue for a while, you increase the pressure and force some of the cellular and tissue fluids into the blood to be carried off, leaving a dent. When you take the pressure away, the fluid comes back to restore the normal balance.
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