This is a "test mode" for most PSRs, Tyros, Clavinovas, Electones, is way to find if something is really WRONG with your Yamaha.
Before you turn on the instrument, in the second octave (from low to high), if is a PSR put C# chord (C#-F-G#) and hold it while turn in ON...(If it is an Electone, do the same but in the lower Keyboard), now when in that mode, with the TEMPO arrows <> go to parameter to check, then with the START(style), execute that particular task, when finishing that, simply push start again to get out, and proceed to the next parameter to check, and repeat as necesary, to get off of that function, just turn off the keyboard and back on.
When test the midi, you must conect a midi cable from de midi out to the midi in...yes, to make a loop.
Do not over use this mode, is design to be use by the service autorized people by Yamaha.