>>Do they select one line when power is applied to a solenoid type plunger, and then the other line is selected when power isn't applied?  Or does a pulse of power switch them back and forth?  If they maintain the currently selected tank when power is absent, it seems to me that this could remove fuel lines and a leak prone selector valve from a lot of cockpits.

The pdf for that product says "remains in actuated position after power
 is disconnected" so it only needs power to change - not to sustain a particular selection.

-Saro