I think it's also related to where you invest your effort.
Assuming you're still trying to just hold on with your arms as your undulations propel you forward, investing your effort when your arms are in front of the shoulders, should use your back and chest muscles. Trying to use strength when the arms are already behind your shoulders will use less chest and back muscles and more arm muscles. What you should really be doing at that point is sliding the arms toward recovery.
I find that imaging you're throwing your shoulders at a forward circular motion and at the same time simply letting go helps with this.