Author's Note: The Seven Potters is an incredibly holey chapter in a rather holey book. I think I can cover several more passages in upcoming scenes.
"The Trace, the Trace!" said Mad-Eye impatiently. "The charm that detects magical activity around under-seventeens, the way the Ministry finds out about underage magic! If you, or anyone around you, casts a spell to get you out of here, Thicknesse is going to know about it, and so will the Death Eaters."
"Oi," said Harry equally impatiently, "who cares about spells?"
"What?" Mad-Eye blinked at him. "Oh, yes - of course, what do you take us for, boy? We'll be using brooms, threstals, and -"
"No, not that! Haven't any of you heard of Vanishing Cabinets? You might have," Harry said delicately, in a voice dripping with sarcasm, "when Draco Malfoy used a pair to let Death Eaters into Hogwarts."
Moody blinked, then shook his head. "Think, boy, think! If we did that, what would we do when they found the one you used still here? They'd be able to come through after you! What would you do then?"
"Blow up the one I used once I was through, and see what they do then," Harry said cheerfully. "Or, since I'd be through and there's no one else underage, the Trace would be gone, whoever brought it could Apparate it away, and the Death Eaters would be left to find an empty house." He twiddled his thumbs. "Or leave it, cluster around the one on the other end, and keep a constant stream of hexes and curses going towards whoever tries to come through. Full of possibilities, really."