The Quincux Formation video explanation!!!

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I don't think the video is actually correct. Sure that's one possible interpretation of manipular tactics, but disengaging while in contact with the enemy is difficult bordering on almost impossible. Even in the lulls between combat, it's a recipe for men fleeing.

look the second video -in the same page-

The problem

The formation presents a problem. Many argue that the legion could not have gone into battle with such large gaps because the enemy would have used them to penetrate the Roman position. Against phalanx formations this tactic could work, but against (Celtic) war bands this tactic would certainly be a disaster; the Celts would pour into the gaps and attack the manipul that stayed in the flank, and would make if almost impossible for the retreating manipul to retreat in order.

In the flash movie at the top of this page I deliberately did not let the phalanx move up. Down here you can see the Celts breaking through.