The theory behind Hood's armour over the magazines

How was the armour over Hood's magazines supposed to improve protection?

It was meant to deflect or trigger shells before they reached the magazine box, leaving the local magazine armour to contain the remaining blast.

The upper layer was intended either to reject an incoming projectile or cause it to detonate before penetrating deeply. Armour around the magazines then provided another barrier between the resulting blast and the propellant spaces. This was a layered theory of disrupting a shell's path rather than relying on one plate to stop every hit. The answer presents the logic of the arrangement without claiming that the scheme made the magazines invulnerable.