What it does
Windows games use Alt for many actions, and a Mac keyboard has no Alt key. Malbec makes the left Option key send Alt to the game, so those actions work without remapping. Store windows, such as Battle.net’s sign-in, never get this, so Option still types characters like @ on keyboards that need it. The right Option key stays as it is.
How to use it
It is on for every game. To turn it off for one game, open its Game Settings › Input and turn off Left Option acts as Alt. It takes effect the next time you press Play.