You are right Eileen. The 'Rotary Speaker' button will affect the fast / slow speed - but the point is, it *only* operates on rotary speaker DSP effects.
On earlier models - e.g. Tyros 5 - every effect program had one parameter marked as the effect variation parameter. (This is quite a different thing from the XG variation effect). In the effect setup you could choose an alternative value for this parameter. Pressing the dedicated DSP VAR button toggled between the 'main' and 'variation' values. While this could be used to switch a rotary from slow to fast, it could also be used with any other effect program for example changing the frequency on a phaser.
That button - and the variation parameter - is not present on Genos, and the function cannot be assigned to anything else.
That very limited function (not well known about by most I'd bet
is presumably redundant, since the firmware now permits all
DSP parameters to be selected for editing and saving at will, so a Yamaha-defined Variation partial workaround is no longer required.
If Denn wants to toggle a particular parameter between 2 values one way to do this could be via 2 registrations. Each reg would save the voice with the associated DSP having the different parameter values of choice.
[So the "Variation button" becomes a couple of registration buttons]
John