Lets break it down
Action Phases
Each Action Phase starts with activating a creature, flipping their action marker, and removing a guard marker if they have one.
Incapacitated
An Incapacitated creature cannot take any actions, including moving, attacking, guarding, casting spells, or counterstriking. It cannot use any Defenses, and if a guard marker is on the creature, it is removed
immediately. A Flying creature loses the Flying trait as long as it is Incapacitated. Any Damage Barrier on an Incapacitated creature continues to function normally. An Incapacitated creature cannot hinder the movement of enemies. Other creatures may ignore an Incapacitated creature. An Incapacitated creature still takes an Action Phase during the round, but it cannot perform any actions during its Action Phase (normally, the action marker is simply flipped over). Conjurations cannot be Incapacitated. Mages are affected by Incapacitation differently. A mage’s mind is so powerful that even if he is Incapacitated he can still work his magic! An Incapacitated mage can still use actions to cast quick non-attack spells. He cannot cast full spells or attack spells.
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Because the Incapacitated for Knock Down goes away after the action is flipped but before you take an action you can still take normal actions when that creature has its action stage.
Remember that the quick cast does not pass back to your opponent so you could Knock Down something then attack it while it does not have counterstrike, defence, or flying. Also another trick is to use Knock Down after the creature has acted so the Incapacitated would last the entire round into the next round. Until that creature has an action phase.
I hope this helps