The idea of the backed jib is to prevent tacking and keep the boat on the same tack more or less at an angle into the wind. As the boat bears off, the main fills and the rudder turns into the wind. As it turns into the wind, the main luffs and loses power and the backed jib prevents a tack. Without the jib it would tack and with the rudder hard over would spin the boat into a run and then gybe.

Lighter, fin keeled boats like my Hunter 35 are less stable that heavier full keeled boats and won't heave to as well. But by carefully deploying a balanced amount of a backed jib with trimmed main it'll heave to OK.