You must never initiate your powerback procedure unless you have a marshall indicating with signals how fast and where to go. also is imperative that both crewmembers have their feet on the floor and NOT on the pedals; to brake you have to take the PL out from reverse and if necessary power up a little to arrest the backward movement.
Here is a little horror story while powerbacking: If anyone of you had flown into Aruba on any high season, carnival, easter, etc.. (that is when you have to hold above the airport in order to get a ramp, because it is so crowded that there is no more space on the ground for you to be)you know how caotic the ramp gets, there are only 17 parking spaces and #12 & #13 are positioned in order for you to be completly downwind, with 25-30 kts winds, so in one of those routinly days groun assigned me #13 and when I was about to enter they called me and instructed me to powerback in (in an ATR 42), I asked for a marshall and they told me to expedite and excersice caution! while powerback on my own, luckily I had another company aircraft waiting on the taxiway next to me, directing me the best he could, my F/O told me that he felt like parking a long truck on a delivery ramp, after that we have to wait for 45 minutes for the pax bus to take the passenger out
