It can be scary to watch. You are playing with your baby, and suddenly they stiffen up. Their hands ball into fists. They might jerk their arm as if they were startled...
The Nervous System Response
Babies do not have the ability to filter out stimulation like adults do. When their limit is reached, their body reacts physically. Clenched fists are a sign of tension, often signaling that the baby is trying to process too much input at once.
What to do next
Stop all interaction immediately. Move to a dark room and use low, rhythmic sounds to help their system reset.