Defensive items indeed activate as a response to an attack. The defensive enchanted items are supposed to be in a state of always ready. When we just look at the rules the wizard even gets to decide if the item auto activates or not. Not sure that this should be possible on any item though.

Making sure I understand this correctly:
If I evoke a Shield myself, it takes a basic action and blocks future attempts to kill me for its duration. If someone attacks me before I raise a shield I have to rely on mundane defence.
If I get a Shield out of a can I can activate it reactively after someone launches an attack and it will protect against that attack, then continue protecting me for its duration.
Is this correct?

If I have a block on someone they have to beat both the block and my defence to hit me, right? Can I activate my block-providing enchanted item after my dodge roll fails, or do I need to use it immediately after they declare their attack?