You choose whether to add or remove counters as the ability resolves
If you're able to fill up your deck with cards that have additional triggers when they die, you can chain these abilities turn after turn to keep punishing your opponents until there's nothing left in play
Among others, these cards included: March of the Machine and Multiverse Legends March of the Machine , representing the journey across the Multiverse and the defense against the Phyrexians, contained a bonus sheet with legendary creatures from across the multiverse
You are never forced to activate mana abilities to pay costs, so if there is a mandatory additional mana cost (such as from Thalia, Guardian of Thraben), you can decline to activate mana abilities to pay for it and hence fail to cast the suspended card, leaving it in exile
This is just as powerful as a modal spell, but arguably better since you can get both halves, not just the best one for that situation