This exercise helps keep balance in the arms for strongman competitors and powerlifters. It's a great alternative to the cheat curl. Set up like a deadlift and use the hips to drive the weight up to a standing position and then lower under control. This is a good way to use more weight than normally used on a barbell curl while sparing the low back, which can be exceedingly overused with cheat curls.
To really benefit from the power curl, concentrate on the eccentric (negative) portion and lower the weight under control.