While the skill description refers to "allies" of the target, this skill actually checks for foes hostile to you. This can include foes that are not allies of the target, such as masterless minions.
If combined with Moebius Strike, you can easily spam this skill, repeatedly knocking the enemy down.
Note that both of the attacks knock down your target so if your target is adjacent to an ally when he was hit by the first attack but is not when he is hit by the second he will still be knocked down. "Brace Yourself!" also prevents only the first knockdown, so your target will still be knocked down, even if affected by this shout.