The best results I've had with the stiff arm are with holding the A button a good couple steps before coming in contact with the defender. I've also had it work when I hit it later, but with a much lower success rate. When you get it out early you can really push a guy off and keep going without much speed lost. If you hit it later and it works you'll lose a lot of speed, just like if you time a Truck stick well you'll keep going fast but if you time it not so well you'll lose some speed.
I want to see some spin move guys! It is such a tough move to pull off, but has the most explosive potential in my opinion.