>>725285568
Activating an effect means starting the chain link. Resolving the effect is when the chain starts going backwards and actually doing all the effects. Anything that happens while building the chain(aka while activating) is not an actual part of the effect itself, but is either targeting or is paying a cost. Anything that happens as the chain resolves is the actual effect.
You can tell which is which by the semicolon. Anything before a semicolon is done on activation so is not part of the effect. Anything after IS the effect so is done when the chain resolves.

Compare MX Saber Invoker to Gigantic Spright. Both detach 1 to summon from deck. But Invoker detaches before the semicolon so is done as cost and happens before the opponent can stop it. Gigantic detaches after the semicolon, so if you were to chain an effect to Gigantic to destroy it on a higher chain link, then it'd never get to detach the material. And since Gigantic says "And if you do, special summon a guy" that means that he did not detach so he does not get to summon.
This is also important with Redoer and Tear. Redoer also detaches as effect, and Tear monsters need to go to the GY from an effect(not a cost!) to trigger their fusion, so Redoer detaching one can cause them to fuse while 99% of xyz monsters cannot.

>>714751406
Would it still involve milling? Something like choose 3 cards from your deck then your opponent randomly chooses 1 to mill and the rest go to the bottom of the deck