Each time a 5★ Hero that isn't the focus of the summoning event appears, a charge will activate. When all three charges have activated, the next 5★ Hero to appear will be selected from focus Heroes only. In this case, the normal 5★ appearance rate will be added to the 5★ focus Hero appearance rate.
If all three charges activate during the middle of a summoning session, the focus-only effect will not apply while using the remaining summoning stones. In order to target focus Heroes only, you must finish that summoning session and start a new one.
The charges will automatically reset if you summon a 5★ focus Hero while all three charges are active, or when the applicable summoning event ends.