You use the faculty (Indriya) to exert the strength (Bala). Say you are a person generally strong in saddha you can use this fact to apply saddha for your spiritual development.
"[note 1150]..These five βfacultiesβ (faith, etc. as 6 (vi)) become powers (i.e. unshakeable by their opposites) from First Path (Stream Entry) onwards.."