I have figured out how it works now:
Fn+F9 = Key repetition speed and delay mode. When you first press Fn+F9, you have to choose between four different delays and repetition speeds using one of Fn+1 through 4 and one of Fn+5 though 8. Using Fn+F9 again will save the selection. Next time you select Fn+F9, one each of 1 through 4 and 5 through 8 keys will be fully lit up to show you previously saved values (which you can change again with Fn+1 through 8 if you want). Depending on which of delay and repetition values you chose, each key you press in this mode will be repeated more or less times after a small or long delay. Press Fn+F9 again to go back to normal keyboard functioning.
I have my keyboard hooked through USB, not PS2, and this does work.