A lot of new humaniod robot builders tend to ignore dynamics and the effects of momentum when they try to create their first motions. They have the robot take its first step, then repeat the exact same sequence over and over to simulate walking. Their stops are abrupt, often so abrupt that the robot loses its balance and tumbles over.
But, humans, and other animals, don't walk that way. They typically gently accelerate from a still standing position to full motion over two or three initial steps. Of course it's more work, but in the long term all the hard work is rewarded. "399"-san created his robots walking stride to slowly accelerate from a stop to full walking motion over the duration of two steps. And, he's also experimenting with a three step acceleration to see if that results in even better and more realistic walking.