In the 1999 anime series, Killua crushed the heart.
At that point, Gon respectfully returned the tag, which Hisoka accepted.
He unleashed a power we never saw during the early parts of the series, which burned the castle down along with the Queen who angered him.
We can definitely see how Leorio improved in that exact moment, as he uses an Emitter-type Nen ability to hit Ging.
His emotional flare causes him to forge a Covenant and Restriction, which gives him the power to defeat Pitou.