A question about Mana Mastery passive tree

In the Mana Mastery node in the passive skill tree one of the choices is

Clarity has 100% increased Mana Reservation Efficiency

Can someone explain what that does? Does the mana reserved double in order to use Clarity or does it drop to zero or what? I can't figure out exactly what happens if if I choose that option in Mana Mastery and tried googling it but still haven't understood what exactly it does.

Any help would be greatly appreciated.