Jboots Mana to cast?

[Evilbob]

When 100% mana and right clicking Jboots it costs 12 mana to cast but 0 mana if not full mana.

[Loly Taa]

Just spammed my jboots while at full mana and didn't see it take any away?

[Evilbob]

Med until 100% Cast once, takes away 12mana on me 51 Necro. But if 99% or casted once already wont take mana away, just when I am 100%