To give a sense of the magnitude of these forces, a hub motor with a 12mm axle making 40 N-m of torque will exert a spreading force of just under 1000lb on each dropout. A torque arm is normally a separate piece of metal attached to the axle that may have this axle torque and transfer it even more up the frame, hence relieving the dropout itself from acquiring all the stresses.
Tighten the 1/4″ bolt between your axle plate and the arm as snug as possible. If this nut is usually loose, after that axle can rotate some amount and the bolt will slide in the slot. Though it is going to bottom out preventing further rotation, by the time this takes place your dropout may previously be damaged.
The tolerances on electric motor axles can vary from the nominal 10mm. The plate may slide on freely with a bit of play, it could go on perfectly snug, or in some instances a Torque Arm china little amount of filing may be essential for the plate to slide on. In situations where the axle flats will be a little narrower than 10mm and you are feeling play, it is not much of a concern, but you can “preload” the axle plate in a clockwise course as you tighten everything up.
Many dropouts have quick release “lawyer lips” that come out sideways preventing the torque plate from relaxing toned against the dropout. If this is the case, you will want to be sure to have a washer that fits inside the lip region. We make custom “spacer ‘C’ washer” because of this job, though the lock washer that is included with many hub motors can often be about the right width and diameter.
For the hose-clamp model, a small length of heat-shrink tubing over the stainless steel band can produce the final installation look even more discrete and protect the paint job from getting scratched. We contain several pieces of shrink tube with each torque arm package.
However, in high electricity devices that generate a whole lot of torque, or in setups with weak dropouts, the forces present may exceed the material power and pry the dropout open. 
When that happens, the axle will spin freely, wrapping and severing off the engine cables and potentially triggering the wheel to fall right out of the bike.
In most electrical bicycle hub motors, the axle is machined with flats on either side which key into the dropout slot and provide some way of measuring support against rotation. Oftentimes this is sufficient.