Board Thread:Help desk/@comment-188.96.106.118-20121222203245/@comment-4937115-20130405024256

If the debuffs are separate it will block the slowing debuff without affecting the AS debuff. If there is only 1 debuff that reduces both AS and MS, it will block it and effectively protect him from the AS reduction as well. Basically it varies from case to case.

Also interesting to note, it completely removes the stacks from Viktor's Gravity thingy, making it impossible to reach 3 stacks and therefore it protects you from the stun as well. Like I said, it varies.