AFAIK there is no electrical switch that controls the motors. This is definitely true on the up position and I'm fairly certain that it's also true on the down position. This is the reason the regulators are bent all to hell. If you force the window closed, all the motor torque goes into bending the regulator frame.

Take a look at the regulator and see if the gear is missing the last two teeth when the window is all the way down. The motor may run fine, but won't engage the teeth in the regulator if they are stripped. It's a common problem due to the power of the motor and it's ability to wear out the teeth at either end of the travel.

Don't leave your finger on the button after the window reaches the end of it's travel. The regulator will thank you for it.