Here's you a cool momentum experiment with some important implications for aikido. Set up a line down which you can walk. Put a finish line 15 or 20 feet down the line. Walk rapidly down the line and stop suddenly with both feet on the finish line. Watch how your body reacts to recover your balance and control your momentum. Typically, you'll see things like:
- elbows out to the side and knees bent
- weight on the balls of the feet
- vertical posture lost - butt out to the back
If you can't see some of these adjustments, walk the experiment faster until you can, then repeat it until you can see the adjustments at slower walking speeds. It takes a while for you to recover from the sudden stop. Watch to see how much time to takes you to recover a neutral posture on the finish line.
Now, try it this way: walk rapidly down the line, place your first foot on the finish line and use the second foot to take a sliding sidestep on the finish line. When the second foot lands, recover your first foot back under your hips. You can use this sidestep to spend all your forward momentum, leaving you in shizentai on the finish line - even from a near-run speed.
The point is, any time you are changing from forward to backward motion or vice versa, there must be a sidestep or else you are left hanging out motionless on the finish line for a relatively long time before you can return to a neutral posture and move again. Stay tuned for some application of this sidestep principle to oshitaoshi and udegaeshi.