Adverse Neural Dynamics (AND), sometimes called Adverse Neural Tension (ANT)
or Adverse Mechanical Tension, probably occurs to some extent in all cases
of RSI.

This is where the nerve (in the arm, shoulder etc.) becomes tethered.
This means the nerve cannot slide in its protective sheath and so causes
pain, tingling, misfiring of the muscles and spasm. The nerve can become
tethered due to a pressure point on the nerve or damage to the sheath it
runs in.

The original cause of the tether could be due to muscles being tight and
scrunched up. This is often the case with RSI and happens because of the
static posture you sit in at a keyboard. As the muscles are tight it
restricts the blood supply to the them and results in them getting fatigued.
Eventually the muscles stay in this fatigued state as the never get to relax
and have the blood supply restored.  Surrounding muscles become tight to
support the fatigued muscles and the problem spreads.

Tight muscles also press on the nerves as they pass through/around the
muscle group, causing a tether. It also results in restriction of the blood
supply to the nerves which prevents them from functioning properly and can
eventually cause the nerves to send pain signals to the brain.