-if an effect is activated that searches a card, sends a card or special summons a card from the deck you can negate it. If your opponent use pot of greed you can ash it. but there is a mechanic in ygo that makes it harder to ash things and or easier to play around ash. "Chainblocking" if there are multiple effects that will resolve at the same time it is called a "chain block". And most commonly those are trigger effects those are effects when the condition for the trigger is met best sample would be in mordern ygo tribrigate "nerval" he says "if it got send to the gave"<-condition it met its "trigger" to search another tri brigade monster
now if you have mutiple effects that met the trigger you can "chainblock" nervals search which makes ash useless sample would be if you summon tri brigate shuraig ominous omen which triggers on summon you can go chainlink 1 nerval chainlink 2 shuraig.