Sadly, Yamaha never joined Korg on the ability to swtich accompaniment memory on/off (you would need to use registrations to accomplish this). But on Korg, if you turn acmp memory on, then it holds until you change chords. But if you turn memory off, it stops playing the backing (except the drums keep going) once you let go. I do wish Yamaha had added that option as well (yes I'm aware of chord cancel, but it's not as intuitive)
Chord cancel on a Yamaha is achieved by playing 1 consecutive semitones (e.g. C, C#, D) and will turn off the acmp, but leave the drums running. However, there's usually a cymbal crash that gets triggered as well, which I find a bit to harsh (ok, it's pretty gentle sounding on the Genos 2; it was really harsh back in the PSR3000 days). It's almost simpler to just turn the acmp button on/off
Mark