The TIP120 side will be significantly more positive than the 5V side. This will forward bias the 1N4001 which will short out the spike, protecting the TIP120 and other circuitry. I measured peaks as high as 100 volts across the Radio Shack motor. The diode that is built into the TIP120 also helps by shorting negative spikes to ground. The large reverse voltage spike seen when power is removed from a coil is often called Back EMF .
Most kids spin the wheel by slapping it.
The more slaps per unit of time, the faster it turns. Also, the longer the hand is in contact with the tire, the faster it turns. Pulsing the motor with the full voltage and current produces the same magnetic force in the armature, but for a shorter time. Thus, the force provided by the motor is about the same. Hit the tire or pulse the motor only half the time however, and the average voltage is cut in half.