An airline must have FAA approval to powerback. Each aircraft, each airport and each gate must be approved and the pilot and ground crew trained in powerback operations. Also, at some gates, powerbacks may be permitted straight back only, no turns. At AirTran, only the dc9 and 717 are permitted to powerback. I don't think any jet aircraft with wing mounted engines has powerback approval.
Powerbacks can't be performed during periods of heavy precip, with a reverser inop, with contaminants on the ramp that may accumulate on the wings (snow,slush) or cause a hazzard to the people on the ramp and the ramp folks must wear eye protection.
The procedure is simple. The engines are started at the gate. On signal from the ground crew and clearance from ground/ramp control the airplane moves forward a few feet. This moves the aircraft off the flat spots on the tires so the thrust required to back up is reduced. The brakes are applied to stop the aircraft and the engines are put into reverse targeting a power setting of about 1.2 EPR (dc9). Once the aircraft gets moving your heels go to the floor to prevent inadvertently applying the brakes, which may cause the nosewheel to leave the ground. To stop the engines are taken out of reverse and forward thrust applied.
It saves time and the need for alot of ground equpt. The ramp area is kept clean so FOD is at a minimum.