I believe he is talking about shielding on the back side of the weld. For example if you are welding something that is sensitive to exposure to oxygen when it is molten like titanium or stainless then you not only need shield gas from the torch but you need shield gas on the back side of the weld also. Regular steel or aluminum doesnt require that.



Ill double check my book when I get home tonight but im pretty sure that is what he is refering to.

Adam