When the satellite orbits around the earth, it does not leave the earth nor fall back onto the the earth. It follows a certain track known as an orbit. How is this track created? It is created by one force pulling the satellite towards the earth, and another force moving the satellite perpendicular to the earth. The two forces must be balancing each other out. If the perdendicular one is too large, the satellite will leave the earth, and if the other force is too large, the satellite will fall back down to the earth. The gap/orbit/track is the one that separates the two forces, in other words, allows the two forces to harmoniously co-exist at the same time. When you are running on that track, you will not feel either force, but they both exist.
When we push hands, we must find this gap. One way to find the gap is to rub/grind 搓/碾 to cause a. We must continue to find the next gap once the one is used up (or we run to the end of the gap). The gap disappears when the opponent changes the posture, the way they apply force, or the timing. We must always match these things with the opponent before we can find the gap.