Progenesis Flask

Progenesis Amethyst flask

I noticed this flask on some builds and am wondering just how it works beyond the "oh I got hit while the flask was up so hit points get taken over time."
Is that just: if I have a stone golem that gives me 100 life per second, then I can offset 400 of the damage over those 4 seconds?

The wiki talked about using recouped life to offset the damage and scaling the flask effect to 300% to offer damage immunity of a sort.

Since up time seems to vary from 7 to 10 seconds, is there a way to keep the flask up on a regular basis?

What does "it is multiplicative with Petrified blood" mean?

What other effects work well with this flask to enhance its use?

Thanks.

That's quite many questions !

1) that's the thing. If you get hit for 1000 damage, then you lose 750 life right now, and another 250 over 4 seconds (63 per second). If you have 100 life regen per second, these 250 damage will be offset and you'll regen 37hp/s

2) if you get 300% flask effect, 100% of damage taken from hits will be lost over 4s, so if you also have 100% damage from hits recouped as life over 4s, you become immune to any sort of hit during flask effect.
However 300% flask effect is way over what's achievable for unique flasks.

3) To increase uptime you want to have longer flask effect duration, flask charge generation, reduced charges used and/or increased flask charges gain. Flask charges generation is mandatory for bossing, but for mapping/farming monsters, a good clearspeed should be enough to sustain your flasks up.

4) Petrified Blood has a similar effect when low life: 40% damage from hits is prevented, and 75% of this (ie 30% of the original hit value) causes a life loss over 4s. (values for a 21/20 regular quality gem)

"Multiplicative" means you apply Progenesis, and then Petrified Blood, and that they do not stack additively.
Example: supposing you're low life and you get hit for 1000 damage.
250 damage is lost over 4s due to Progenesis
40% of the 750 remaining damage, ie 300 damage, is prevented by Petrified Blood, and you start losing 225 life over 4s (30% of 750, considering a 21/20 gem)

All in all you lose only 450 life immediately, and 475 life over 4s that you can offset with recoup and/or enough recovery.

5) There are severay ways to use Progenesis at its best, here is mine : try Pathfinder. On top of having access to all "increased flask effect" of the passive tree and have some in ascendancy, she offers easy charges sustain, and a huge life recovery with life flasks, that comboes with Petrified Blood (as you can recover above low life with flasks) and ensures you can offset most of the life loss over time even with minimal recoup.

Awesome answer! Thanks very much.