Torque arms are used to prevent axle rotation in hub motors. When a hub motor is powered up running, for all the torque that the motor generates spinning a wheel forwards, there is an equal and opposite torque on the axle causing it to rotate backwards.
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. However, in high power systems that generate a lot of torque, or in setups with weak dropouts, the forces present can exceed the material strength and pry the dropout open. When that happens, the axle will spin freely, wrapping and severing off the motor cables and potentially causing the wheel to fall right out of the bike.
To give a sense of the magnitude of these 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 is a separate piece of metal attached to the axle which can take this axle torque and transfer it further up the frame, thus relieving the dropout itself from taking all of the stresses.
For forks that don't have fender eyelets, or where the fender eyelet holes don't line up, we created up updated variant of the original hose-clamp model with the Rev3 layout. Here, there are small slits for 2 or even 3 hose-clamps to provide extra spinout strength, and the long curved slot in the axle plate allows for for the clamping arm to attach parallel with the fork tube and always intersect correctly.