Trash should be used allot as a sin tank to proc your auto crit shock. That being said you should be using wither and discharge off cooldown and should always be working towards 3 stacks of harness darkness for the awesome channeled heal from force lightening. That being said just like every SIN spec in the game you should be using the free attack when you are below 20 force.
Thrash is a TPS and DPS loss.
It should ONLY ever be used when you're going to be wasting Force regen (e.g. Wither, Shock and Discharge on CD, less than 3 stacks of HD, single target, and Force at ~70).
You maximize DPS, TPS, and survival by just using Shock and Wither on CD to build 3 stacks of Harnessed Darkness ASAP, filling in empty GCDs with Saber Strike (except in the above corner case), and using Force Lightning immediately upon reaching 3 stacks.
Using Thrash is so Force-negative that you'll wind up delaying your use of Force Lightning to hunt for Energize procs, and Thrash doesn't do enough damage to justify using on its own. Consider Energize procs happy accidents when you're trying to avoid wasting Force regen, and don't make any effort to make them happen.
So basically, priority winds up looking like:
Dark Ward (if 1 stack remains, or remaining duration = ~1s)
Force Lightning (with 3 stacks of Harnessed Darkness)
Discharge (to maintain debuff)
Wither (on CD, basically)
Shock (on CD, basically)
Thrash (Shock, Wither, Discharge on CD, <3 stacks of HD, >= 70 Force)
Under ordinary circumstances you'll toggle between Shock > Wither > Shock > FL and Wither > Shock > Wither > FL, averaging one FL about every 12s (the two patterns build HD at differing rates).
Please do not call Noxxic.com amazing site (unless by amazing you mean amazingly stupid)
Now granted, if you look just the first part of the rotations there, it is fairly good and can be of some good help for someone new to the class, but that is all.
I would not recommend Noxxic.com for anyone ever.
Almost everything else they say about assassin tanks is completely wrong!
(and every single guide for every class contains noteworthy mistakes)
This. This so hard.
Noxxic is a terrible resource, and does little except to get new players onto a path of mediocrity.
Not a single one of their guides is accurate or describes how any class should gear, spec, or play.
It was put together by a bunch of scrubs too dumb to know they're scrubs, based on oudated information, and no effort has been made to update anything on that site based on any empirical evidence, theorycrafting, or evolving metagame.
What good is duplicity for a tank? I mean, how often do you maul to make it worth the points?
Worthless in PVE, definitely worth a point in PVP when you can actually circle dance.
There's really no reason to ever deviate from the standard
31/0/10 build at 50 for PVE tanking.