Wim is correct. Before any changes can be made to various Genos settings, they must first receive the "focus." With a knob assignment, you're achieving focus and changing tempo all in one motion, which doesn't work.
Yamaha was brilliant in placing the Tempo buttons on the left of the display window. With the Chord Hold function on and during a 4 bar phrase that has the same chord (for example), you have time to press one of the Tempo buttons to achieve the focus. The tempo increases (or decrease) by one beat - hardly noticeable
*. At this point, you can move the speed up or down by small amounts with individual button presses or a lot using the wheel. I also believe there is a setting where you can force the Tempo window to remain displayed for a longer time, should you need extra time to "hunt" for the correct tempo while making a chord change.
I've used this on occasion in a live duet setting. Sometimes our "rehearsed" tempo is a bit too fast or slow. The singer wants to apply a slightly different interpretation to the song. I use the Tempo buttons to make minor adjustments to get us back in sync. In a live band situation, the drummer and bass player will do the same thing, if they're sharp. Luckily, we can achieve the same thing with the Genos
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*EditIn a live situation where the singer is creating tension by being out of sync with the Genos, I've often used just a one beat per minute change to get us back in sync.