http://www.youtube.com/watch?v=NNlu_wA-VWA
After watching this clip again, I have found something interest. In the beginning, both persons were pushing forward. When the Sumo guy pushed a bit harder, the Chen guy yielded, redirected, changed his pushing to pulling, and dragged the Sumo guy to the ground.
The Sumo guy might not know that he could do that too. What if the Sumo guy started to pull before his pushing (if pulling is allowed)?
If the Chen guy could used pushing to set up his pulling (left hand push on his opponent's right shoulder toward him). The Sumo guy should be able to use his pulling to set up his pushing.