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 each dropout. A torque arm is definitely a Torque Arm china separate piece of metal mounted on the axle that may take 
this axle torque and transfer it even more up the frame, as a result relieving the dropout itself from bringing all of the stresses.
Tighten the 1/4″ bolt between the axle plate and the arm as snug as possible. If this nut is definitely loose, then axle can rotate some quantity and the bolt will slide in the slot. Though it will eventually bottom out and stop further rotation, by enough time this takes place 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 may go on properly snug, or in some cases a little amount of filing may be necessary for the plate to slide on. In scenarios where in fact the axle flats will be a bit narrower than 10mm and you feel play, it is not much of an issue, but you can “preload” the axle plate in a clockwise route as you tighten everything up.
Many dropouts have speedy release “lawyer lips” which come out sideways and stop the torque plate from sitting toned against the dropout. If this is the case, you will want to be sure to have a washer that meets inside the lip location. We make custom “spacer ‘C’ washer” because of this job, though the lock washer that comes with many hub motors is normally about the right width and diameter.
For the hose-clamp model, a small length of heat-shrink tubing over the stainless band can make the final installation look even more discrete and protect the paint job from getting scratched. We incorporate several pieces of shrink tube with each torque arm deal.
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 strength 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 correct out of your bike.
In most electric bicycle hub motors, the axle is machined with flats on either side which key into the dropout slot and provide some measure of support against rotation. In many cases this is sufficient.