One evening, while reading Ulysses, or maybe Game of Thrones, I realized my foot had "fallen asleep." I decided to look into what was happening with that. You can play along by sitting on some appendage while reading this post really slowly or multiple times.
The Condition
The technical term for this sensation is paresthesia, which sounds a lot worse than "pins and needles". We'll just look at the temporary or transient form here, which results from temporarily cutting down the blood supply to the nerves, or compressing the nerves themselves. If you have lingering, tingly feelings that are not love, you should see your doctor to have it checked out.
The Nerves
Nerve cells send and receive signals to and from other neurons in the brain and spinal cord. If these cells are squashed, the chemicals that form these signals can't move around properly and the communication breaks down.
Blood work
Nerve cells need a lot of oxygen, glucose, and other nutrients to function. If the blood cannot reach them with these supplies, then they will not function properly.
Feelings
The results depend on which nerves are affected. If it's the nerves used for sensing that stop working, then the area will feel numb. If it's the nerves that tell your muscles to move, then you won't be able to move that part. Also, some nerves stop firing, while others may fire hyperactively or erratically. Your brain interprets these mixed signals as burning, prickling or tingling, a little like static on a radio.
Relief
When the pressure is released, the blood can supply the nerve cells again, and the nerve signals can fire more regularly.
Recovery
But you might notice that this doesn't all start working immediately. At first, the pins and needles feeling can feel worse. While the blood flow is limited, nerve cells increase how well they absorb nutrients. When normal blood flow returns, they may then overreact.
Rates of Return
Different kinds of nerves recover at different rates. Nerves for pain and temperature recover fairly quickly. The motor control nerves recover more quickly than the ones for sensing touch, so you can move parts before you can feel with them properly.
Do you have any observations to share about pins and needles?
References
What makes your Arms, Legs and Feet fall asleep?
What is Paresthesia?
What Causes Arms and Legs to "Fall Asleep'?
What is Pins and Needles?
Why do we get 'Pins and Needles'?