By installing a vertical guard just to the inside of the small chainring, a chain that overshifts past the ring will stop at the guard and get guided back to the ring.
It hangs off the front derailleur fixing bolt that the derailleur utilizes to secure itself against the front derailleur braze-on or hanger. If you use a braze-on-style front derailleur and a clamp that secures that to the frame, it is compatible with that as well.
