register_event only invokes on key press.
register_switch is a toggle.
You should use register_axis like this:
this.register_axis("car/controls/power", {minval:0, center:1000, vel:1000, acc:1000}, function(v){ movePitchDown = v })
Vel defines the speed at which the value changes during key press, you could also use Number.POSITIVE_INFINITY there instead of 1000 to guarantee instant switches, but in reality nothing is instant so you should have there a number that says how fast it switches. A value of 1000 means that it changes from 0 to 1 in 1/1000 sec.
Center tells how fast the value goes back to 0 upon button release. If it's zero, the value stays where it was and you need a second key binding with negative mode that can push it back. Also, if you only have one key bound to such an action, it automatically becomes a toggle.