Yamaha P-100 Owner's Manual page 27

What about the split point key?
The key that you press to select the split point will always be included in the main keyboard
area. Thus, if you select F4 as the split point and then set the main keyboard below this point,
the main voice will play all notes up to and including F4, and the sub voice will play any notes
from F#4 on. If you set the main keyboard above this point, however, the sub voice will play any notes up to E4,
and the main voice will play notes from F4 up.
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To return the main voice to the area above the split point, the
second key you press should be above rather than below the split
point. The dot in the display will move back to its normal position
before the number.