Sitting in the same position for a prolonged period causes compression of nerves in those body parts subject to pressure – usually your hands and legs.
Because nerves are responsible for transmitting electrochemical impulses that aid communication between body parts, when those nerves are compressed, communication to the brain is temporarily disrupted.
“When you start moving after a long period, connectivity is suddenly restored, causing the prickly sensation we describe as pins and needles”
An analogy to understand this would be a garden hose that when twisted restricts the flow of water . Similarly, when the nerves of a region are compressed, transmission of electrical impulses is restricted. As a result, the brain has no way to interpret what sensation the region is experiencing.
This happens most commonly in the lower extremities because fibular nerve that goes around the knee gets directly impacted when people sit with their legs crossed for long periods of time. When you start moving around again, pressure on the region is relieved, restoring sensation and causing the prickly sensation we describe as ‘pins and needles’.