From the Rogue Trader rulebook, page 107, for the Talent Takedown:
If he hits and would have done at least 1 point of Damage, then ignore the Damage and the opponent must make a Toughness Test or be stunned for 1 Round.
Looking elsewhere, the 'normal' character does 1d5−3+SB damage. (SB = Strength Bonus). Damage is reduced by armor and Toughness Bonus, though, so it is unclear in which cases the Takedown fails. Obviously, if the damage roll is 3 or less (assuming a SB of 0), it fails. But what if it is a 4 or up, and the opponent has a Toughness or Armor reduction? Does the 'would have done at least 1 point of damage' test occur before or after that reduction?