Shield Enchantment: Intangibility

This is a simple idea for a shield enchantment (which in general, I want to see more of). Intangibility (temporary name) goes onto a shield. The effect is that when you crouch / activate the shield, you become a spectral version of yourself, unable to be touched or harmed by mobs. 

This may seem overpowered at a glance, however, by default, mobs cannot harm you with a frontal attack when you are using an unenchanted shield. Additionally, there would be natural balances.

Mobs could still SEE you and try to attack, crowd around, creepers would still explode near you, destroying surroundings. Essentially, while you are taking advantage of your intangible abilities, you would have to be crouching, meaning mobs could still crowd around you and rush you, likely creating an even worse threat than the one you were momentarily fleeing. 

Intangibility would just be an emergency situation defense that delays being killed, or could provide the opportunities to escape, heal, etc. There are obviously a lot of balances to be considered, but I think that an enchantment which, when activated, makes the player invulnerable but slow an unable to build / break would actually be pretty fun and tactical. Just an idea.