Followthrough is a new mechanic that pays attention to the stack. Followthrough can be found on Instants, spells with Flash, and activated abilities. The template is always "Followthrough—When you [cast ~ / activate this ability] in response to another spell or activated ability,...". So you would want to cast the card or activate the ability in the same way you'd play a counterspell, when another spell or activated ability is currently on the stack, before it resolves.
This mechanic essentially gives a little bonus whenever you trigger it, though the card itself already does something without triggering Followthrough. This can get interesting and tricky, because you may start playing cards at sub-optimal times for the advantage.
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For instance, Opt Through is basically Opt, which you often want to do on another player's end step, but it's like Opt and a half if you cast it, say, as your opponent casts a creature during their main phase.
Call the Volantile would ordinarily be a card to play after an opponent declares attackers, but you might give up the surprise for an extra token.
Triggering the Followthrough ability on Seer Savior gets tricky, because you'd most definitely want that surprise First Strike after attackers are declared, and it's not likely that the opponent is going to play an extra spell or activate an ability before blockers are declared. But you aren't stopped from casting your own spell or activating your own ability first, to act as a cantrip, to get that trigger.
Just such a trick can be pulled off even more easily with Flash of Lightning, which is Shock when you don't Followthrough, and Lightning Bolt when you do.