To give a sense of the magnitude of the forces, a hub motor with a 12mm axle generating 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 attached to the axle which can take this axle torque and transfer it even more up the frame, hence relieving the dropout itself from taking all of the stresses.
Tighten the 1/4″ bolt between your axle plate and the arm as snug as possible. If this nut is usually loose, then axle can rotate some quantity and the bolt will slide in the slot. Though it will eventually Torque Arm china bottom out preventing further rotation, by the time this happens your dropout may already be damaged.
The tolerances on motor axles may differ 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 little amount of filing could be necessary for the plate to slide on. In situations where in fact the axle flats will be a lttle bit narrower than 10mm and you are feeling play, it is 
not much of a concern, nevertheless, you can “preload” the axle plate in a clockwise path as you tighten everything up.
Many dropouts have speedy release “lawyer lips” which come out sideways and prevent the torque plate from sitting toned against the dropout. If this is the case, you will need to be sure to possess a washer that fits inside the lip region. We make customized “spacer ‘C’ washer” for this job, though the lock washer that comes with a large number of hub motors can often be about the right width and diameter.
For the hose-clamp version, a small length of heat-shrink tubing over the stainless band can help to make the ultimate installation look more discrete and protect the paint job from getting scratched. We include several pieces of shrink tube with each torque arm package deal.
However, in high vitality 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 electric motor cables and potentially leading to the wheel to fall proper 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 offer some measure of support against rotation. In many cases this is sufficient.