some mics are dual impedence. You can use the positions called hiZ with a normal high impedence (1/4" on one end) and the mic will work at max levels. The only time it would benefit you to use the low Z input is if the mic input on the mixer or keyboard accepts low impedence signals. The Yamaha keyboards do not! The benefit would be that if the Yamaha's had a balanced XLR mic pre built it, then the low Z level could be cleaner and with more gain, but since this is not the case, a hi z mic or low z mic with line transformer would both be the same. Using a low to high z transformer just turns the loz z mic into a high z mic with more gain.
George Kaye
Kaye's Music Scene
Reseda, California