This was the best picture I could take, thanks to the fact that the flash on my camera is pretty far from the lens. But you can basically see what's going on - there is nice, even penetration from the weld bead on the inside of the head tube. You might also notice that the bead is shiny and silvery, which is due to the fact that I pump argon (which is inert, if you remember your high school chemistry) inside the frame while I weld it.
This doesn't make the weld stronger, per se, but it eliminates the oxidized crap you'd otherwise find on the inside of the weld (that crap can create stress risers that reduce the longevity of the joint) and the big plus is that it saves a TON of wear and tear on my reaming/facing/chasing tools - things don't distort as much, and instead of hardened, oxidized crud on the inside of the head tube (or bottom bracket, or seat tube) that has to be reamed out, I have a nice smooth weld bead and almost no need for reaming. I've probably done 300 frames on my current set of reamers and taps without sharpening them once. That's a HUGE savings, especially given that the argon to backpurge the frame only costs a couple of bucks (I run the backpurge at 5cfh).