Is it like a regen boost (like stamina regeneration, only while not using), or will it add back like health regeneration (in set intervals ignoring, if it is used right now)?

The first one. It multiplies the eitr you are already getting back, the way Brimming does, so the game's one second pause after a cast still applies and what changes is how fast the bar fills while you are outnumbered.

The other version would hand eitr back during a cast, and that is a much stronger card than this one is priced as. Good question to ask before it is built rather than after.