I noticed the sims only use soul barrier with the condition “seconds to big hit” but it’s a huge semi oh shoot button as well. I say semi because it’s strength is dependent on how many souls you already have and for me I find I often have time to ensure I can get more souls by using a fracture of two first.
The reason it’s better to have the souls going into it immediately with using it is because a “soul-less Soul Barrier” has a pretty low absorb effect so you can lose the entire effect of it before the very next GCD or two, never being able to build the absorb higher. But a 4-5 soul soul barrier will have a much larger absorb initially which increases the chance to build the absorb even higher by keeping the buff/effect/absorb up longer.
It’s of course very huge for anticipated big hits even without souls as all absorbs are but I get so much more use out of it than just that. It’s a pretty short cooldown, only 30 seconds. I don’t have a robotic thought process on when I use it but basically I’ll replace a 4-5 soul spirit bomb with it if I feel I need to relax the healer or my health is uncomfortably low where a good spirit bomb wouldn’t be enough (maybe sub 30%). And I’ll use it even without souls asap if it’s all I have available and I need anything I can get to stay alive.
I’m speaking mainly for m+ here as raid fights are much easier to predict incoming damage.