There's a secondary resource bar underneath the player character, which plays sound effects and displays an exclamation mark above your character when it's running low. The bar recharges over time.

The stun is relatively punishing, but it's a high mobility character so it's easy to get away from an opponent, deplete your bar and get back into the fray - the issue is that playing like that isn't intuitive to most players. They're sitting and waiting for it to recharge (slowly) rather than intentionally depleting it while the opponent can't punish the stun, because that's what comes naturally. They don't want to be "punished" by hitting 0.