Warrior Shield Slam Glyph

Recently in the arena priests, druids and monk healers have been bugging me as an arms Warrior. My mortal strike says it reduces healing, but whenever I get a healer into danger a massive shield often pops up and while I break it down, he's healed back up again.

I've taken Shattering Throw to deal with Paladins and Mages using invulnerability to heal, but I'm considering trying Gladiator stance and a Glyph of Shield Slam as it says it removes one magic effect per slam.

Can anyone tell me how this magic removal effect works? If a priest uses a shield spell on himself, will the glyph kick in first removing the shield before the damage is applied, or first the damage of the shield then remove it if anything is left over.

Also is it random what magic effect it removes, or does it work on a last in first out basis?

I've looked it up on wowhead but most of their comments are about the old version of glyph that gave 10% more slam damage