Hi, and thanks for the reply. The purpose of FALLING is to notice when the switch is pressed, pulling the pin to the ground. Therefore, no interruption is triggered, when it jumps back up as the motor rotates away.
Right now I'm pretty sure the problem is that the Motor Shield actually uses the D3 pin for PWM speed control. Therefore, when the pin is pulled down, the speed becomes zero, and thus the motor is not actually moving.
I think I'll just do this without interruptions.