Video Link
Link: Pulling body back and forth...not pushing

    MOMENTUM V. INTERNAL 
     aka Momentum Force vs. Internal Energy

Master Stephen Hwa told me when I first met him that the Yang Style, Wu Style forms I learned were "rounded Square Form" as is the universe of Tai Chi out there...I had no idea. Here's that "rounded" square form in operation: You decide to drive down the highway alternately and indiscriminately pushing your foot on the accelerator and the brake You would be using your engine to give the vehicle kinetic energy/momentum then throw the energy away by pushing the brake, over and over. You would consume much less fuel if you only drove steadily...think "Round" form in ClassicalTai Chi. Master Hwa, a Ph.D. Engineer, talks a great deal about momentum force, aka pushing forward with the back foot, pushing back with the front foot, referred to as "other" Tai Chi and all are "rounded" Square form...look for yourself on Youtube. All of these contribute to a movement that has "stops and starts" in it, one way or the other. When it comes to Classical Tai Chi "round form" using a "pulling" coupled with internal energy, not momentum and that difference is a really big deal. One might say that Tai Chi relying on "pushing with a foot" is extravagant of energy, whereas "pulling with a foot" truly stores it and releases it only when needed.