This is a combination of the Inside Arm Only and Outside Arm Only drills. The inside hand should be on the oar during the drive, and the outside 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 only be done on the square (since the inside hand is not on the handle during the recovery).