Activation base is in Stamina and values vary with fatigue%.
The character strikes twice in rapid succession when activating this talent, both for normal combat damage. It is thus much stronger than expected. In fact, this inconspicuous ability is the best damage dealer for a two-handed weapon warrior throughout early-middle levels. It's a cheap, short-cooldown, first tier double swing with a bonus of reducing enemy attack for a while. One or both attacks from Sunder Arms are capable of scoring critical hits.
Sunder Arms applied penalty doesn't stack with itself.