TTB takes all damage dealt (except status effects) to one of the enemies, and deals 35% of that as action skill damage to all linked enemies.
So say we deal 1000 dmg to one of them. 350 dmg will then be dealt to all others – however since it’s elemental damage, those modifiers apply. E.g. we’re using fire as action skill element, that will then deal 350 × 1.75 = 612.5 damage against flesh and 350 × 0.5 = 175 against shields.
Now prior to the patch this was considered action skill damage for damage calculation, i.e. you could increase it with Do Harm. Let’s say we have 5/5 Do Harm (that’s +4.5% per stack) and 30 stacks, that’s a bonus of 4.5% × 30 = +135%. Thus, we’ll now deal 612.5 × (1 + 1.35) = 1439.4 against flesh and 175 × (1 + 1.35) = 411.3 against shields.
With 99 stacks and 3/3 Awakening you could get a total bonus of 4.5% × 1.3 × 99 = 579.2%, so TTB would deal 612.5 × (1 + 5.792) = 4160.1. Now let’s add +30% fire damage from Tempest and another +10% from the elemental damage guardian rank: 4160.1 × 1.4 = 5824.1.
We just hit a skag for 1000 dmg and 5 other skags nearby received 5824 dmg each. Even with fire against shields every linked target would get 1783 dmg, so still far more than the original target. If it’s a splash weapon and it hit 3 of the enemies, each of them would deal that damage to all others. That’s how stupidly overpowered it was and why it was so superior to all other of Amara’s action skills, which did pretty much zero damage on M10. It was pretty broken.
So now they increased all action skill damage in the game on Mayhem levels and things got really out of hand. They increased TTB less than the other action skills (×7 dmg on M10 instead of ×31) but it was still absurd: it means we’re at 40,768 dmg now from that 1000 dmg shot. I believe they realised this during testing, so they “fixed” it before releasing the patch by removing the ability to increase it with Do Harm.
Now it’s tied to your Mayhem level, so it’s 35% dmg on M0 (with no option to increase it via Do Harm) and 245% on M10 (which is more than that maxed out situation before, but without spending any skill points). It would have made more sense to just keep it how it was and don’t do any Mayhem scaling on TTB. That said, they mentioned in the patch notes that this was a temporary solution, so this may be changed again.