A failed knockdown roll can cause “stun,” as can certain critical hit results and some afflictions. If you are stunned, you must Do Nothing on your next turn. You may perform any active defense while stunned, but your defense rolls are at -4 and you cannot retreat.
At the end of your turn, you may roll against HT. On a success, you recover from stun and can act normally on subsequent turns. On a failure, you remain stunned; your next maneuver must also be Do Nothing, but you get another roll at the end of that turn . . . and so on, until you recover from stun.
Mental Stun: If you are surprised, you might be mentally stunned; see Surprise Attacks and Initiative (p. 393).
This sort of stunning works as described above, but you must make an IQ roll, not a HT roll, to snap out of it. You’re not hurt – you’re confused!