One arm the hard way places the unused arm by your side, you will rotate your upper body from side to side for every stroke as if you were using your unused arm. The hardest part is you will breathe to the opposite side of the arm that you are using. One arm the hard way challenges your ability to rotate in the water because you have to create the rotation you typically have in your stroke with only one arm. It makes breathing difficult and requires one to breathe quickly and then snap the head back into place because you can’t “hang out” forever rolled up on your side without an arm leading. Without a straight arm leading to steer you through the water the drill makes you pull in a way that just propels you forward and does not create excessive torque that causes the body to twist or steer to one side or another.