There is a window of inbetween being too much and not being enough Oxygen or Air for the roots.
If they were 100% exposed to air, for a long time, like all day, then decay (root rot) would start.
If they were 100% submersed under water, for a long time, like all day, then decay (root rot) would start.
Seo we seek that window in the middle .
When in VEG, AFTER the roots are in the water, if we Drain the tank, and expose the roots to an AIR bath for ten to 20 minutes, they will get a large dose of oxygen and we will see a growth spurt in height. If that Air bath lasts too long, then we get bad results.
That Air POCKET is a way of spliting the difference 24/7 and keeping that WINDOW open.