Here is an on line description of what I am talking about.
Eliminating Oxygen and Nitrogen at the weld start to reduce the possibility to weld porosity and excess weld spatter requires good shielding. In addition to Oxygen and Nitrogen the surrounding air contains Moisture or Water Vapor. The Water Vapor can also be drawn into the arc where it will disassociate into Oxygen and Hydrogen. Hydrogen can cause more than porosity problems. Some amount can dissolve in the molten steel and will only come out when the weld cools. Since they are very small, Hydrogen atoms it can migrate through the steel accumulating near defects, dislocations, etc. forming Hydrogen gas. This can cause cracking. These cracks may be in the weld itself or in the adjacent parent metal called the heat affected zone.
Well, where does the hydrogen embrittlement/ checking come from with MIG welding?
Using the same principles of inadequate inert gas flow to sweep away gasses that will react with the hot metal, you get the same effect in the area just behind where you welded. So as your welding along, the gas is directed exactly where your welding now but NOT where you just welded. The area you just welded is still very hot, hot enough to break apart the moisture in the air into hydrogen and oxygen which interacts with your weld, since there isn't any flux to prevent the hot metal from coming into contact like you do with stick welding you end up with some hydrogen checking/ embrittlement.
The reason why its a little less likely with TIG is that you go slower, but some metals especially the exotics need that post flow for a reason, to prevent hydrogen embrittlement!
Am I making any sense or still smoking crack?
Don't get me wrong, its most certainly possible to get hydrogen embrittlement with stick, you get some of it with 6010 because the cellulose flux has some moisture in it, and if you try to weld on sub freezing steel with 7018 you will most certainly get hydrogen embrittlement.
Anyways, not wanting to be arguemenative, just trying to express what I feel is usefull information.