Learn flag out in wingfoiling: let the wing fly without pressure, hold it at the front handle or wing tip and fly on swell without pull. In 5 steps.
The honest answer: flag out means holding the wing so it no longer produces pull and still flying. Then only the wave carries you, not the wind in the wing. It is less a trick than a posture. Instructors describe it like this: you hold the wing with one hand on the front handle (the pistol grip) and let it hang at the side, or you grab the handle at the end of the leading edge and let it fly like a flag [1]. Because the wing then no longer pulls, you feel the foil and the wave alone.
Why that is useful is explained by the same tutorial: if you ride swell and keep the wing in pull all the time, the wave pulls you away. Advanced riders keep the wing ready at the front handle so they can quickly bring it back into pull [1]. Also you see the wave better when flagged out because the wing is not in front of your face [1]. That is practice knowledge from an instructor article by a retailer, not a study.
Flag out means: the wing is a tool you hold ready, not a line you hang from.
The IKO lists "catching swell" as the last stage of its wing foil chain [2]. Flag out is the technique that carries swell riding. Realistically you need many sessions for it (experience value).
Hold the wing above you so that it does not pull: flat to the wind, slightly raised. First learn on land how this feels.
Milestone (practice): you hold the wing neutral for 30 seconds without it pulling.
Take the wing with one hand at the front handle, the pistol grip, and let it hang at the side. That is the starting posture [1].
Fly at flight height and set the wing neutral without pulling. You notice at once how much pull you used to take from the wing.
Milestone (practice): 10 seconds flying with a neutral wing without touching down.
Grab the handle at the end of the leading edge and let the wing fly [1]. That way you can fly without pull. The wing stays held so that you can pull it in again in a flash.
Use flag out on a bump: depower the wing, look at the wave, bring it back into pull when the wave fades. Hold the wing where you can see the wave [1].
Milestone (practice): 3 bumps in a row with flag out without the foil touching down.
Wingfoil injuries mostly arise from contact with your own equipment, and the most common cause was riding errors when fatigued, at 77.5 percent [3]. Flag out takes concentration and strength: whoever holds neutral works with trunk and legs. Take breaks in good time.
If the wing falls into the water it can suddenly develop pull. The leash keeps it with you, but you should be able to lift it immediately. Practise on open water with company and wear helmet and vest. This guide explains and motivates, it does not replace a wingfoil school or supervised training.
In flat water try "wing off, wing on" in alternation: ten seconds of flag out, then back into pull, ten repetitions. The switch trains exactly the moment in which flag out fails on the wave: getting quickly back into pull. If you manage that cleanly in flat water, it will work even better on the wave.
You hold the wing so it does not pull, for example at the handle at the end of the leading edge, and let it fly like a flag [1]. That way you fly carried only by the wave. The description comes from an instructor article, not a study.
On the wave a wing in pull often pulls you away from the swell. With flag out you use the energy of the wave [1] and see it better. It is the technique behind swell riding, which is the last stage of the IKO wing foil chain [2].
After swell riding. The IKO lists "catching swell" after foot switch, jibe and tack [2], and flag out builds on it. There is no fixed time frame.
It is no more dangerous than normal wingfoiling, but you are at sea and often tired. In the wingfoil survey 77.5 percent of injuries came from riding errors when fatigued [3]. Take breaks and wear helmet and vest.
Linking Bumps in Wingfoiling: Glide From Swell to Swell