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Line of sighting means moving so that terrain is between you and whatever you are LOSing. In that context, the tank probably goes around a little corner after aggroing the mobs, so they all stack up on the corner for an easy AOE burn.
It's used more often by nonDK tanks, as DK tanks can just chain the mobs into a clump instead.
Sometimes it is far more cost/time effective for the tank to run a second or two ahead to pull everything around a corner and stack them up that way than to try to chain everything to one place. It's something you'll have to determine by how spread the adds are at start and the make up of ranged vs melee.