Odometer digital test display - hows it work?

Saw a short vid on Y tube last nite, where the guy showed how to put the odometer display into a test mode, and one of the modes was a digital tach, I thought that was pretty cool. But the vid did not go into any of the other 15+ tests ( what they are, what the read out is ).

Perhaps one of the real Guru's here can elaborate or point to a more comprehensive vid or webpage?

BTW, once the car is shut off and re-started, it goes back into normal Odo display.

Odometer digital test display - hows it work?