Just got it straight from the Horse's mouth on LA (and it was EM Acies that called Mesanna a horse - I'm just quoting him).rolleyes:
Mesanna showed up, and when asked, she said that Spell Consumption was a chance of one of the two following events occurring (As in it could be EITHER ONE, at random; not sure what the odds are of a. as opposed to b.):
a. The Attacker's spell being blocked.
or
b. The Attacker's spell being converted back to mana (inferred to be added to the protected person's mana pool, as sort of a defensive mana leech).