It’s mostly useful to detect hardware issues, but here’s how to use it: press PROG, LED and left Fn together at the same time to enter the test mode.
First, it will test the LEDs by going through a few colors (waiting for any keypress in between): red, green, blue, then white. It will follow that with 1000 cycles of rainbow.
Then it enters the matrix test mode. In this mode, when you press a key, it will aggressively check for chatter (by reducing debounce time significantly). By default, all keys start out red. Held keys turn green. If chatter is detected, they turn red. If everything is ok, it turns blue.
To leave test mode, you will have to reach the matrix test phase, and the same combo you used to enter it, can be used to exit.
This is what it does at the moment.