Regarding a Poe Ninja build I found, can't figure out how it works

Here is the build from user xalxixinhaDELVE : https://www.pathofexile.com/fullscreen-passive-skill-tree/AAAABAABAQDBBbUGdwjUDPIPxBBiEcIZihqNGo8dqh3CHvAfWyLqI18knST9LL8tqC3EL2831Dt8Ps9GcUcGSOVQQlFHUilTMVM1VeBYWlsmW29iWmNDbAhwUXBScLtzbXZve257136wf8Z_-4MJg_GEU4TFhweJ04w2jb-P-pMnlwaXIZu1ndmgQ6EvoqOkFqXLpq-m-KxGsZCyKLQMtMW3dbfWuMq5k73mvoC-isF8wcXC7MMJwzrEosrT0NDUI9hU2RPbWdvn35jhiOdU6Nbr9ewY7T_v_fD58YryHfLh9tr7Cf_e

he has Pain attunement + CI, and has barely any increase to ES%, how does it work?

update: Just realized it's the unique belt. However, another question has emerged, why does he take perfect agony?

next time, post the poe.ninja link so we can actually see what's going on. A tree without items is not much use :P

He probably uses timeless jewels.

Judging by the character name, I assume this is a deep delve build which are very specific for that content