It triples your fSTR bonus for the strike as well.
I believe that it's calculated like this: occasionally, it takes whatever damage you would deal--prior to phalanx and stoneskin calculations--and triples it. Though I haven't exactly done a lot of testing, so the damage might be post stoneskin/phalanx.
Best way to test this: ballista with a errant'd/bio'd brd or rdm with mandau vs a rdm with max phalanx. The damage should be single digits, then once in a blue moon, leap up to the 30s range--well beyond triple.
Proc rate is indeed between 3-5% however it will only proc on the first attack in a given combat round. Therefore, if you offhand mandau (or guttler, or kikoku, etc) for whatever crazy reason, you will never see it proc. This is most apparent with Spharaioffhand never triples despite using the same knuckles as mainhand.
Similarly, if you double- or triple-attack with the weapon, only the first strike of the 3 can multiply (and I'm not positive it can trigger on the same combat round as a mainhand multi-attack...screenshot plz).
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