When I worked ramp for AA, we were taught powerback procedure, however, we never actually did one, since we had ample tugs around.
The only aircraft that AA allows to powerback are the 727, md-80, and F-100 (if so equipped). We almost had to powerback a 757, however since we could not find a towbar that worked. After numerous calls, however we managed to find one that fit at UPS (there are 2 different towbar attachments that boeing made - one that could be attached by a 727 towbar, and one that was fit over like a clamp 737 style. AA had the 727 style, and DL, UA, HP, and OAS (another contractor at ONT) only had the clamp style.
Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.