This is a combination of the Inside Arm Only and Outside Arm Only drills. The outside hand should be on the oar during the drive, and the inside hand on the oar during the recovery. The hand that is not actively holding the oar can be laid on top, so that it is ready to grip the oar at the changeover.
Note
This drill can be done either on the square or on the feather (since the inside hand is on the handle during the recovery, when the blade is first feathered and then squared).