The Cannon doesn't directly translate into fighting. It's just illustrating the way momentum from a larger Mass can be transferred to a smaller Mass. Where you see it is as he demonstrated it, using a unified whole body Mass to move a "disconnected" piece of the opponents structure, an arm or the torso, pulling the structure off balance and making it very easy to tip them over or throw them out.
Or in a proper taijiquan punch if you need to go there.
Or when practicing "receiving energy"