Link to interview with Ondrusek:
He addresses the well-publicized fact that Tom Brown taught him the cutter and its impact on his career. He also talks about the struggles in the Fall League.
If his cutter is actually the same speed (or slightly faster as he says) as his four-seam fastball, it is no wonder that he is effective. A cutter that comes in looking exactly like the fastball and then moves at the last moment would have to be impossible to recognize and hit.