Trapping Argon around the bottom of the DT to HT junction. The angle here is tighter than it looks in the picture. With the foil there and a few seconds of "pre-flow" (actually achieved by arcing off the foil very briefly and waiting for the post-flow as my machine doesn't have a pre-flow) the region fills up with enough gas to mean you can take some big liberties with the amount of stickout. It also helps to put foil in the end of the HT because splitting the flow of Argon on an edge like that always seems to cause problems. This is probably the second hardest part of the frame to reach after the CS to BB junction which also always needs foil