[IMG]http://i1061.photobucket.com/albums/t464/TheMelonHead/Diagram_zps157qsh8x.png[/IMG]

As you can see from this crudely drawn diagram, when the spear was initially stopped by the chain-link so sufficiently that there wasn't even enough of the point remaining to even pierce Beatdown's skin, there was still a significant width of blade behind the chainlink, as is altogether common with a spear. When the spear was subsequently lifted up with no small amount of force and retracted, the widest part of the blade would be set against Beatdown's groin as it cut upwards, the angle would further diminish any protection the chainlink could provide. 

My diagram is shit, but the shaft of the spear would be angled downwards, as would the point, as Brennus aimed for a lower target. Hence when he retracted his spear, that same angle would be used in reverse, ensuring the width of his blade cut Beatdown before the chainlink played any part. 

Addendum: The spearhead is actually a leafblade, I just painted it like a triangle because it was easier.