A full flag requires strong anti-rotation strength, which signals deep rooted and stable core muscles.
It also builds great physical, mental strength (patience, dedication, consistency) and mobility as we practice it.
Like handstands and other isometric poses, there are countless little things to adjust in order to hold a "clean" version of the humanflag.
One of the most thing has to do with anti-rotation work during the hold, to prevent the hips and torso from pivoting upward.
This bias naturally occurs since it is an "easier" way to hold the pose.
Here is a very simple and good way to train for anti-rotation to activate obliques and other useful muscle parts.
It’s called the “side way deadhang” hold from a high bar.
You would start in a normal dead hang, shrugged up (aka lats engaged) and straight legs, toes pointing down.