This effect seems to work somewhat different from the tooltip.
At level 1 it should block 30% of all damage with mana until below 50% mana, but it actually blocks 100% damage with mana until below 50% mana. have not tested the higher levels. It feels like this is stronger than intended.
Edit: see comment further down for more details, also changed the number on the "below % mana value from 30% as I first wrote, to 50%.