system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does your skin turn red when pressure is applied for long periods of time?
gpt
In short periods of pressure your restricting blood flow causing your skin to turn pale/white. Once pressure is relieved you get a rush of blood flow making the area look flush/red. This is called reactive hyperaemia. 

In longer periods of unrelieved pressure you get non blanchable erythema I.e persistent redness despite relieving pressure. The prolonged lack of blood flow and oxygen results in blood vessel damage and  other cellular processes.  As a result your blood vessels remain dilated and even leak blood into surrounding tissues. This causes that persistent redness. Tissue will recover if pressure is completely relieved, if not it can result in a pressure ulcer!