When you are done with your save rolls and are about to remove models you can choose an ardboy equipped with a shield and make two singel rolls. If those roles are not a 6 that model is dead and you can continue allocating wounds. If you rolled a 6 you can roll another one and see if he dies.
Another example. You take 8 damage and you have 3 ardboys with shields in the unit of 10 models and none of them are damaged since earlier. You make 6 rolls since they have 6 wounds (you cant roll more than the boys have wounds). One of the dice rolls a 6. 2 of the shieldboys die and 1 is left with 1 wound. You now still have 2 wounds to allocate. You roll 1 dice to see if you save this one too, You dont and he dies. the single last wound is allocated as usual to a model.
Important to note: Doing the above and then removing models without shields would be cheating.
Also, you can accidenty remove models and break coherensy. This is why you always consider the placement of the shieldboys when charging and piling in. Just keep them together in the back and if you by accident have them mixed in with the others, just remove something else instead, you are not forced to use the shields and as of the new book they have the same attack profile as the rest of the boys.