When your forearms turn rock hard on a descent and your fingers can no longer modulate the brake lever. Not a strength problem but constant tension without recovery.
Arm pump isn't a minor complaint, it's a safety issue: a rider who can no longer modulate the brake is riding at the edge of their control — and hand and wrist are among the most common injury sites anyway. The cause is almost always a death grip plus too much weight on the hands. Every remedy attacks the same point: hold the bar only as firmly as you need, take load onto the pedals through bent elbows and a central stance, set the levers flat and in line with your forearm so one index finger reaches them, and let suspension and tyre pressure absorb the hits. On calm sections deliberately straighten your fingers, and ride sessions with real breaks. The caveat: you reduce this over weeks, not in one session. If it stays extreme and painful regardless, see a sports physician.