He is referring to the hand of the balance (non-throwing) arm. I believe it is to help keep you arm from rotating to far towards your back which would cause your shoulders to over rotate as well. To see this work first stand up and stick your balance arm out with your thumb pointed up. Now, move your arm back as far as you can. Next, do the same thing with your thumb pointed down. You will find that as your hand gets to be even with your shoulders, you will feel resistance. This resistance keeps you shoulders from over rotating during the release.