So it works like Malzahar shield. Malzahar shield breaks if he gets hit by an enemy champion, epic monster or large monster (blue and red buff) but Malzahar cannot rebuild back his shield, unless he doesn’t take damage for an X amount of time and if he does take damage the shield is reset.
Now Yasuo has a permanent shield with 0 counterplay. In the late game that is like a 600 shield for free every few seconds because it is so easy to charge it. Yasuo shield flow charge doesn’t even reset if he takes damage so basically he gets a windwall and a self shield that has 0 counterplay that will block 600 damage for free without risking anything.