The error in Chen’s treatment is that there can be no incoming force by the opponent without an equal-and-opposite reaction force (forces always come in pairs). Thus, the reaction force, which would be exerted by the defender, would be exactly as large as the incoming force. But the main idea is for the defender to exert minimal force in dealing with a powerful attack. Nevertheless, Y. K. Chen’s useful method of analysis will be employed in Section 6 to treat parts of Treatise 7—here using the appropriate quantity, incoming momentum, instead of incoming force.
I think he's being a bit pedantic there and missing the point that the movement of the "opponent" is an expression of an unbalanced force. The equal and opposite reaction is between the ground and their feet propelling them into you. When we connect up, then we can balance the forces to get to that zero Wang Peisheng was talking about and then we've got their feet, our contact, our feet, and we take their feet away.
I think it's a good starting off point but it really only considers one aspect of using the circle. We've got all 8 of those gates to use.
