Isolation: Submission is based around the notion of isolation. Your goal is to isolate one limb from the other (right leg from left leg, left arm from right arm) and then from the torso. Left on its own, a single isolated limb is easily overcome.
The further a limb is taken away from the torso and from its opposite limb - the weaker it becomes. This is how stronger opponents can be defeated by weaker opponents. If you can consistently pick and isolate a limb and get it extended out and away from the torso, victory via submission will often come your way