Reaction
The target changes. You press F or J, or you press nothing. The drill logs whether you were inside the window and how many milliseconds the good presses took.
How to run it
Use a keyboard. F and J sit under the index fingers on a normal desktop layout. On a phone the on-screen keyboard is a poor substitute, so this family is desktop-first. Wait until the letter is actually on screen. A press in the gap between trials does not count, and a press after the window is a miss.
The score is the share of go-trials you hit, minus a penalty for pressing on X. A perfect run with a slow median is still a clean run. Chasing a lower millisecond count by guessing early will add misses.