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From the information you guys have given me, I typed up a picture of what I assume is happening when a diode is added to the circuit. The circuit shows a P-channel mosfet switch.
It appears that when the positive voltage output is stopped abruptly by the P channel mosfet turning off, the induction from the motor produces a negative voltage on the wire that was previously receiving a positive voltage from the mosfet. The voltage across the mosfet (source to drain) increases ABOVE the battery voltage because the drain wire (motor's positive wire) is now NEGATIVE. The excess voltage across the mosfet will likely blow it without protection. The diode shunts any NEGATIVE voltage on the drain wire (motor's positive wire) to ground. By shunting the negative voltage on the motor's positive wire (mosfet drain wire) to ground, the mosfet is protected.
Let me know if I'm thinking correctly on this.
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