The flag is due to the pull-pressure balance a form of the libra.
The flag in handstand tends very large shear forces to generate. therefore, it is for the base necessary to a very stable pressure point to offer. Unfortunately is not a base so stable as a pole …
Since the pressure point is always below the pulling point, the 2 possible pressure points while the base is standing are the lower leg from the back or the thigh from the front. Thus the only possible pulling points left are the shoulder or the hand.
Once the flyer gets twisting, the strain of his wrist in the pressure point (due to the huge leverage effect) is very high. Therefore, the base should follow the flyer with the pressure point and correct only with the pulling point.
A commonly shown trick is the segue from the flag into the handstand. For this, both hands are gripped and the base puts the deeper hand on his thigh. Then the flyer is pulled into the one-armed handstand onto the thigh and with a little force pushed onto both arms to Hand2Handstand.