I’m not sure how to trigger it while I’m playing on my Yamaha. I’m not sure what you mean by using an on/off switch to “actuate it”
I thought, maybe you used the Yamaha as a solo sound, and the harmonies weren't needed during those sections. The Ketron (and probably all others) has an option for a foot switch on/off control to engage the harmonies. I was referring to that feature. If you use auto acc, or sequences for the Ketron, you can dedicate your mic to follow those changes, and it won't matter which keyboard you play your lead sounds on during harmony singing. Give me a call to discuss, if you'd like.215-284-1328 USA