In this case? Resonance. 

A trump card is "an ultimate X", be it a technique, attack, or innate ability. If he has an ultimate passive ability, it can be fired whenever. In this case though, it was fired in response to the activation of Resonance.

Also you do have to contend with the issue of it not being Stein's heart, but I'm sure BB will explain what is meant by that, so I won't bother.