A torque arm is recommended for hub motor installations that are more than 500 watts (36V with 20A or 25A controller), or for any installation done on aluminum forks. It slides over the non-wire side of the motor axle, engaging with the 10mm axle flats to prevent the axle from spinning out inside the dropouts.
We now have a full range of 7 torque arm models in production that together fit almost all front/rear 12 and 14mm axle motors, along with special ones for regen braking use. We also have motor-specific arms offered here.
For more information on proper torque arm installation and other details see the torque arm homepage.