To torture another metaphor, it would be the difference between slowly letting the air out of a balloon, and popping it. Though the dam metaphor is more apt, what with the excess magic flooding outward.

David looked seawards along the river. He stared, rubbed his eyes, and stared again. One of the rocks seemed to have podded into something swollen, black and smooth.

Fans want to see the Twelfth Doctor go to the 51st century to visit River in the library.

A bruiser with a sandpapery voice, he was a reliable teller of corner-boy tales and, it seemed, a willing cog.