Can someone please explain (beyond repeating the description given in it's sub text) just exactly what it does? Preferably giving an example of where & when you might choose to use it to gain any advantage.
Someone on Steam said it's about boosting your bludgeon, pierce & slash resistances, but I can't see how, 1, they're 'applied', & 2, how they can be 'applied' for varying lengths of time.
As I personally found the Earth Elementals to be quite tough enemies, I was thinking that their decoctions would be worth having. However, without understanding where & when to apply it, It's pretty useless.
Please enlighten a poor confused old gamer. Thanks. :)

It increases resistance versus dot-effects, i.e. burning, bleeding and poison. The longer the effect persists the stronger the resistance gets.

It's pretty useful against enemies, who apply long duration bleeds or burns, e.g. griffins with their 20sec bleed. Vs poison you are usually better off with golden oriole.

Right, so I think I get it now. Trouble is, how could I know upfront whether an enemy applies these things? Get the feeling it's pretty useless, which is a shame, coz as I said, I figure Earth Elementals to be tough enemies. I think the entire decoctions repertoire is lame. Most of them seem either too specific or duplicate bonuses. I thought the Griffin one might be good, but it doesn't significantly protect me it seems. The Relievers is good for Wraiths but I mostly use the NoonWraith one, as I can actually notice a benefit from it. :)

Ekhidna is probably one of the most powerful, especially if combined with minimal usage of whirl, crippling blows (for long term bleeding damage) and murderous precision (for up to a 6% instant kill chance per hit, with a very high rate of blows). Razor focus is useful for the latter, as well as an 'emergency' store for the use of whirl.

I tend to use normal fast attack/dodge with a Quen shield (no special bonuses). Once you have the monster pattern, point straight at your target, dodge to the side then forward if necessary, and apply short bursts of whirl... before dodging away.

If you got hit, then move around until you can recast Quen, which will also add back health. Then rinse and repeat. A dodge at the end of the whirl is quite important though, unless everyone died.