He is talking biomechanics.
If you’re going from knee height to the ground, the shortest way is straight down, but, if you quickly swing your foot out front, your foot will hit the ground further out, but, it will still hit the ground at the same time as dropping your foot directly down. Therefore, all you did by swinging your foot out front, is, increase stride length.
The constant in this, is Gravity.
Hopefully that explains it?