Pressing Power in Drive

At the slightest movement, it does nothing (tried it once while doing 3 or 4mph.)

Luckily...

 

I thought I read somewhere if you press and hold the power button while the engine is on, it will disable the electric engine.

 

I read that too, in the manual. The key is that you have to hold it in. If you just bump it or press it quickly it won't do anything.
Not that I'm going to attempt it....

 

I've done it. Holding it for maybe 3 seconds appears to throw you into ACC mode - the dash goes blank, the light on the button goes green, the engine cuts out, and you lose power steering. Braking didn't seem too badly affected - I didn't really notice.

As a possibly unhelpful quirk, the drop to ACC mode also turns the headlights off! This may be different in the US - here in Europe turning the car off noramlly turns off the headlights, leaving the position lamps on. Presumably turning the switch off and on would have reactivated them.

The state it leaves you in is interesting - you're in ACC mode but in N. The gear indicator showing N is the only thing left on the otherwise blank dash. You can't move to D or R, and as soon as you move to P, the gear indicator on the dash vanishes, leaving you in normal ACC mode.