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.
Hold the cleanest active dead hang you can then bring both legs together up to a side, even just a few inches and hold that pose.
Aim for 10 sec hold each side (x 4 or 5 sets) and improve how high you can hold your feet up session after session.
If you do it right this exercise should feel very difficult and 10 seconds should feel like a tough challenge. Add more time if needed!
From there you would transition into vertical bar work, to transfer and apply the anti rotation work to the horizontal plan!