Hi npapenfus,
while holding the MULTI PAD CONTROL [SELECT / SYNC START] button, press one of the MULTI PAD CONTROL [1]–[4] buttons to put the Multi Pad playback in Standby mode. The corresponding button will flash in red. (You can even do this for two, three, or four Multi Pads at the same time.)
Then memorize this state to a Registration button, and finally save the Registration Bank.
When you call up such a Registration later (that contains one or several Multi Pads in Standby mode), the Multi Pads will NOT run directly on pressing the Reg button, but they will run when you play a Chord with your left hand (when ACMP is on). When ACMP is off you can press any note on the keyboard to start the Multi Pad(s).
To cancel Synchro Start, while holding the MULTI PAD CONTROL [SELECT] (or [STOP]) button, press one of the [1]–[4] buttons.
(Parts of these text are copied from the Tyros4 Owner's Manual, page 65.)
Hope this helps!
Best regards,
Chris