"Skip welds"...I do not claim to state that my answer is the the only answer, or even the right answer, for that matter. These are my recolections.
...back in my day...

...the early frames with no gussets, Schwinn Stingrays and the like, would break right at the headtube.
The earliest BMX frames started with the 'single gusset' at the head tube. (From here on I am going from personal experience.) My first BMX frame was the infamous Pedalpower/ Rink Raider. It began it's life with a single gusset. Then a crack developed on the down tube just past the gusset. Steve Rink took the frame back to the designer/builder (Steve Johnson) and they welded dual gussets on the side of the front.


I never had another problem with cracks in the front end again. All this took place around mid 1975.
It was about this same time that somebody decided that by 'skipping' sections of weld on a gusset it would spread out the pressure and stress points of that critical area.
In hindsite I also like bmxerpete's idea;
QUOTE
To avoid making the tubes brittle due to too much heat from a continous weld
I am NOT a metalurgist or a design technition. I just remember the industry people talking about this stuff back then.
Oh yea, by the way, I rode a Webco (with redline forks, Motomags and box bars) for a short time back then.
