To give a feeling of the magnitude of the forces, a hub motor with a 12mm axle producing 40 N-m of torque will exert a spreading force of slightly below 1000lb on every dropout. A torque arm can be a separate piece of metal attached to the axle which can have this axle torque and transfer it further up the frame, thus relieving the dropout itself from choosing all the stresses.
Tighten the 1/4″ bolt between your axle plate and the arm as snug as possible. If this nut is definitely loose, in that case axle can rotate some amount and the bolt will slide in the slot. Though it will eventually bottom out preventing further rotation, by the time this occurs your dropout may previously be damaged.
The tolerances on engine axles can vary from the nominal 10mm. The plate may slide on freely with a lttle bit of play, it could go on flawlessly snug, or sometimes a small amount of filing may be essential for the plate to slide on. In situations where in fact the axle flats will be a bit narrower than 10mm and you feel play, it is not much of an issue, nevertheless, you can “preload” the axle plate 
in a clockwise path as you tighten everything up.
Many dropouts have speedy release “lawyer lips” that come out sideways preventing the torque plate from resting flat against the dropout. If this is actually the case, you will want to be sure to get a washer that meets inside the lip area. 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 right width and diameter.
For the hose-clamp version, a small amount of heat-shrink tubing over the stainless band can generate the final installation look more discrete and protect the paint job from getting scratched. We include several bits of shrink tube with each torque arm offer.
However, in high ability devices that generate a whole lot of torque, or in setups with weak dropouts, the forces present can exceed the material power and pry the dropout open. When that occurs, the axle will spin freely, wrapping and severing off the electric motor cables and potentially triggering the wheel to fall right out from the bike.
In most electric bicycle hub motors, the axle is machined with flats on either side which key in to the dropout slot and offer some measure of support against rotation. In many cases this is sufficient.