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There's waiting and there's actually waiting.
If there's an attack over the summit of a climb and Froome has a mechanical, waiting may mean just taking it easy on the descent and pootling on the next one until he gets back on. At that point, domestiques have regrouped and while he's burnt from getting back to them, they're fresh to launch a relentless series of attacks. That sort of thing.
It's unlikely but if it happens, they're going to hit him as hard as they can. 1 second is all they need by the close of play on Saturday.
He's still only holding a relatively short margin over the rest of the top 10. A minor crash or a mechanical could see 3-4 minutes disappear like sand through a collander if it happens at the wrong time and place. He'd not unassailable yet.