One common mistake that's made about Hiccup and Toothless's relationship is that Hiccup controls Toothless. This couldn't have been further from the Truth. Toothless generally listened to Hiccup because he liked the Viking, and because said Viking was better at coming up with battle plans than he was. That doesn't mean Toothless is bad at it.

So when Roland manages to shock him in the hindquarters, Toothless quickly realizes that if Roland's back there, it means he's not right in front of the heart. Toothless twists, turning towards the heart, and fires a plasma blast, blowing the organ to smithereens.

Hiccup takes advantage of the hearts exploding, and directs the cord to make another attempt at wrapping Roland up.