HERE'S the Waterford poser . . .
You leave Tadhg de Búrca to sweep with impunity and he reads the game so naturally as to make your team's passing deliveries seem predictable and aimless.
If you press up and mark him, Waterford's rangy forwards weave through that left over space and threaten goals.
For 35 minutes of the All-Ireland quarter-final, Dublin looked to have had it sussed.
Ryan O'Dwyer occupied de Búrca. Chris Barrett, wearing number four, spent more time in Waterford's '65 than Dublin's in his role as personal chaperone to Kevin Moran.
Dublin led by one at half-time but when Waterford dropped yet more bodies even further back thereafter, it left a three-on-three the Dublin half.
Meaning - fatally for Dublin - not only did Waterford's snipers have copious of room in which to do their thing, it afforded neither of Dublin's defend-in-front markers, Cian O'Callaghan or Paul Schutte, any easy pass when they emerged with possession.
"If you're free you have to work twice as hard," says Paul Murphy, who may well find himself in precisely that role on Sunday in the All-Ireland SHC semi-final between the teams.
"The dynamic of not marking a man is that you should be able to get on more ball, and you're not likely to have a lad skinning you."
Murphy looks the best bet of the Kilkenny full-back line to play the role of spare man, even in the absence of Jackie Tyrrell.
"It's one of those things you know will probably happen during the year," he says of the injury to his defensive colleague.
"And it's a matter of who it happens to.
"It was unlucky for Jackie to happen ten days before an All-Ireland semi-final, it's unfortunate for him but we have to get on with it."
Murphy has played there before and his sweet-striking style makes him a creative entity in defence and thus an obvious candidate for sweeper.
"It means reading the game, where the ball is going to drop," he notes.
"If you see a ball coming out of the other team's defence you have to read where it's going - 'this is probably going to land on our '45, I've to be over there and if I don't win the break I've to get in there and make sure the opposing forwards don't get a run in on goal'.
"That's the dynamic, you've to cover as much ground as you can. A lot of it is drifting, that if someone has the ball you're on hand to take it off them to clear it.
"If you're marking a man you know your job," Murphy points out.
"It's to follow a man and to stop him scoring, to limit his influence on the game.
"If you're spare that's gone, you can pick up as much ball as you want.
"It's six of one, half a dozen of the other: I don't mind, but if you're the free man you have to impose yourself on the game that bit more."
Imposing themselves is exactly what Waterford have done in every game they've played this season.
"You can't not be impressed with Waterford this year, they're league champions and they were unlucky in the Munster final," Murphy points out.
"When Dublin put it up to them the last day they had leaders to stand up.
"They've just been so consistent the last while. It was tough in Thurles the last day with the rain that came down but they still looked impressive.
"They did what they needed to do and did that very well."
"So," Murphy concludes, "we'll have to be at our best to beat them."