To give a feeling of the magnitude of the forces, a hub engine with a 12mm axle producing 40 N-m of torque will exert a spreading force of just under 1000lb on each dropout. A torque arm can be another piece of metal mounted on the axle which can take this axle torque and transfer it further up the frame, thus relieving the dropout itself from taking each of the stresses.
Tighten the 1/4″ bolt between your axle plate and the arm as snug as possible. 
If this nut is loose, after that axle can rotate some volume and the bolt will slide in the slot. Though it is going to bottom out and prevent further rotation, by the time this happens your dropout may previously be damaged.
The tolerances on engine axles may differ from the nominal 10mm. The plate may slide on freely with a lttle bit of play, it may go on perfectly snug, or occasionally a small amount of filing may be essential for the plate to slide on. In situations where the axle flats are a bit narrower than 10mm and you are feeling play, it isn’t much of an issue, nevertheless, you can “preload” the axle plate in a clockwise course as you tighten everything up.
Many dropouts have quick release “lawyer lips” which come out sideways and prevent the torque plate from resting flat against the dropout. If this is the case, you will want to be sure to have a washer that meets inside the lip region. We make custom “spacer ‘C’ washer” because of this job, though the lock washer that is included with various hub motors is normally about the proper width and diameter.
For the hose-clamp model, a small length of heat-shrink tubing over the stainless band can produce the ultimate installation look more discrete and protect the paint job from getting scratched. We involve several pieces of shrink tube with each torque arm offer.
However, in high electricity devices that generate a lot of torque, or in setups with weak dropouts, the forces present may exceed the material durability and pry the dropout open. When that happens, the axle will spin freely, wrapping and severing off the engine cables and potentially causing 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 in to the dropout slot and provide some measure of support against rotation. Oftentimes this is sufficient.