I've had the opportunity to retest some of the Lethality mechanics. Read on.
My tests focused on basic mechanics rather than a precise mathematical understanding.
Tooltip Interpretation: Does weapon damage on the initial hit. Increases the damage from "Poison-classified (internal)" DoTs by 30%/tick with a limit of 10 procs (i.e. 10 ticks) on your target. There is no specified internal cooldown on this effect. "The Agent" (you) should be the only one using ticks from WB.
In-Game Results: Does weapon damage on the initial hit. There is no internal cooldown on this effect. Poison applied by other Agents currently uses ticks of WB.
I tested this with another Operative in our guild by applying WB to a target, having him apply his Corrosive Dart, and then seeing if WB lost stacks. It did. I am unsure if this effect is exclusive to Operative/Sniper poisons (they share similar poison abilities) or could be replicated with the "internal damage" abilities of another class (i.e. Sorcerer's Death Field, Force-Internal damage rather than Tech-Internal damage). I consider these results interesting but by no means an exhaustive test of WB.
Tooltip Interpretation: Does weapon damage on the initial hit. Does internal (yellow) damage on the following/bonus ticks. Additional ticks are dependent on number of Tech-Internal DoTs (poisons are the only abilities you have with this damage profile) applied by you, personally.
In-Game Results: Works as designed. Cull's additional ticks also use up ticks of WB. The Tech-Internal damage profile of Cull's additional ticks is probably being processed as "poison" by the game and therefore applicable to WB as well.
I tried having an Operative apply Corrosive Dart and then Culled
that target without applying my own DoTs. I received no bonus internal (yellow) damage on
Tooltip Interpretation: Increases the damage of poison (Tech-Internal) damage effects, cast by you, on your target by 5/10/15% when your target is below 30% health.
Likely Results: Because Cull's bonus damage consumes Weakening Blast, that damage is also likely impacted by this skill. Without a combat log it is difficult to test for certain. Anyone who has played a Lethality variant spec can tell you how monstrous your damage gets once a target is below 30% health.
I will be updating my Lethality guide to reflect the results of this testing.