The two small moons (the inner one is Pandora, the outer is Prometheus) shepherd the F ring (keeping it narrow). The shepherding can be understood as follows.

The inner moon moves faster than the ring particles and so drags them forward (speeds them up). The tends to whip the inner ring particles outward. The outer moon moves more slowly than the ring the particles and so drags them backward (slows them down). This tends to cause the outer ring particle orbits to decay. The inner and outer mooon work at loggerheads; one tries to drive things outward and one tries to drive things inward. The competition leads to the focusing of the ring, taking it from a fat state to a more confined, narrow state. This focusing and confining also occurs in the Uranian ring system,