From NPR:

The hockey-puck-on-a-rod test was invented by a Michigan high school student.

It works like this: Tester suspends the device while injured athlete sits with forearm on table, fingers loosely circling the stick. Without warning, tester drops stick. Athlete grabs stick as fast as possible. Place where athlete grabs gives an instant readout of reaction time.

It all happens in milliseconds - too fast to measure with a stopwatch. Athletes with concussions had reaction times that were 15% slower.