A flag is a form of Libra in which the flyer will be at 2 contact points balanced (with pulling and pressure point). These contact points form a triangle to each other, in which the upper body of the flyer hangs sideways. Giving the impression of a living flag. The flyer absolutely extends both knees and keeps the upper foot (= pulling point) strongly tightened.
Again, the base creates the balance and the flyer has to trust him. The base takes care in the balance that the flag does not unintentionally shifts away sideways. He is managing it with the upper leg of the flyer.
The usual position of the base for the flag is on the thigh while he is kneeling or standing… the lower leg of the flyer is placed on the thigh, the upper one either in the neck or held with the hands of the base.
The professional version is the flag in the neck of the base while he is in the handstand … the upper leg is thereby clamped in between the legs of the base. As I said … the professional version!