PAS on the Fusin kit I am reviewing here:viewtopic.php?f=4&t=39877
simply has a bracket with a hall sensor that is secured to your BB by the BB retaining ring or locknut. It can be put on either side; I chose the easier left side (no chainrings to get in the way). The magnet ring just slides onto the crankshaft.
But you have to take the crank off to install it. Most PAS appear to be made taht way (Justin has a thread around here for working out a different way to make them that wouldn't require that).
When pedalling, it causes the controller to run the motor, even if you don't have any throttle input (or don't ahve one attached). So if you put a signal from a 555/etc into that signal wire, it is going to automatically give you whatever level of power the PAS system is programmed to give, even if you ahve throttle at zero. I don't kow if all controllers do that, but this one does, and I thik generic "infineon" ones do that too.
Anyway, if I don't pedal, and jus tuse throttle, it still works. Also works without the PAS connected, so you could easily put a switch on the PAS signal wire to disconnect that wire from the controller, and have throttle still work, at least on the Fusin controller. Most "generic" controllers are probably like that.