6.5 amp motor tripping 20 amp breaker
Based on picture and follow-on comments-
If it only trips the breaker with the switch insert, and runs without the switch in the circuit, you must be wiring the switch incorrectly.
From your picture, the black should be one side and the white on the other side. You have the wires going side to side, the black wires should be arranged vertical on one side and the white vertical on the other side based on the color of the terminals on the switch. 

The Q&amp;A area for the switch states that the black goes on one side and the white goes on the other side, not on top of each other.  

You will get continuity on one side with the switch on, and no continuity when the switch is off. You will get no continuity from side to side, with switch on or off.   

Please note, even if it work now, you may have damage the switch contacts (internal) by shorting the switch. I would replace with a new safety switch or with a normal light switch, switching only the black(hot) wire.