This is a summary of the theory I had about aura having shear thickening properties. The thread can be found here: https://rwby.fandom.com/wiki/Thread:638683
I believe aura has shear-thickening properties. This means that the viscosity of a non-newtonian fluid increases with pressure, like oobleck (cornstarch and water). This means that with low speeds, Aura can be passed through easily, but at extremely high speeds, it acts like a solid.
Obviously, Aura is not actually a fluid, but by treating it as having similar principles to shear thickening fluids, a lot can be explained.
"Aura does not exhibit true non-newtonian properties, but it does have a few similarities."- Phantom
I think as aura levels drop, the ability of aura to harden like a nonnewtonian fluid increases as adrenaline levels increase. This makes it more likely to deplete as more and more energy is put into maintaining the hardness of aura, and would also explain why aura shatters- it becomes too brittle to act effectively.
As aura levels drop aura is focused on strengthening leg muscles so they get slower as a result. The amount of aura depleted remains the same, but is distributed towards defense, hindering mobility and making them more likely to take hits.
"Nora was able to make a 3 story vertical jump while fighting the Nuckelavee AFTER hers went down; so I don't think that's what happens. More likely Aura is not 100% effective at preventing ATP buildup in muscles so they do fatigue, just more slowly."-Phantom
The ability to heal damage done by carbonic acid is redistributed, therby reducing stamina
"More like their bodies can eliminate it and repair the damage better than a normal human body, but as with their other superhuman traits (Strength, speed, and reflexes) they do have their limits."- Phantom
What disproves Monty had the intention of making aura have non-newtonian properties:
"Aura acts VERY similarly to the Overshield ability, to the point where it looks nearly identical."-Embellem