I'm pretty sure it's something like every swing in a WS has a chance to DA with the trait (might work differently with dual wield?), just that you can't excede 8 swings. I've had a few 19/20/21 TP Guillotines (17 is normal) doing 1.5k with no buffs and 2.5k with Souleater, and same applies for Raging Rush (highest doing 2.4k with 2 DA), etc. In all those cases, the damage obviously supports the numbers, and there's no other way to justify it.
For a while I was skeptical of it myself about it being able to happen, but after reading some extensive tests and doing my own I'm pretty sure the above is true.
Back on topic though, True Strike is so damn broken.
http://img292.imageshack.us/img292/1144/ffxi656cx8.jpg
No buffs other than songs (no SV)... I've been messing around with it on high defense mobs where Rampage sucks, and it's crazy. I popped out a few last Dynamis-Xarcabard for 1-1.4k with the average around 700-800, versus Rampage averaging like 400 damage on the higher end mobs there.