Justforfun000,

This feature currently exists on the SD1, and all X-series via the use of the KEY START and KEY STOP buttons - (depending on if either one or both are on/off)... hope I didn't confuse you on that last part!
With the KEY STOP only on, if you play the chord and release immidiately, the arranger will stop; however if you play and hold the chord for a number of seconds (programmable in UTILITIES) before release, the arranger will keep playing!

Thanks,
AJ