About the article discussing trapped flux pockets (creating voids), I had trouble reading this because english is not my first language. I think they're saying that a cap of solder is created all around the entry of the fitting, so the flux can't escape. They also said to put the flame slightly ahead of the solder and move the flame and solder around the joint (I tried this already but maybe I added too much solder too quickly thus creating the cap all around).
Article:
Flux pockets that are trapped but not
burned — exhibiting wet, shiny copper
surfaces (Figure 4) — generally indicate
failure to maintain the proper relationship
between the point of application
of the heat and the application of the
filler metal. Maintaining the point of
application of heat at the base of the fitting
cup, slightly ahead of the solder
metal is essential in preventing trapped
flux pockets.
Applying the heat behind the point of
application of the solder, or keeping the
heat in one place as the solder is applied
around the joint allows the solder to create
a cap at the face of the solder joint,
through which the flux cannot escape the
joint and becomes trapped. Remember, a
trapped flux pocket will always prevent
solder from filling that space.