This excerise has a rare combination of core stability and dynamics.
The one leg stand with the barbell is difficult on its own, tilt over very slow with the swing leg always inline with the trunk and the hips level. With this exercise you must bent the support leg a little, you need it to push of later.
The swing-forward-lunge. You can't easily command your leg to go forward but a build-in reflex of your body will assist you. Once your body feels it is falling over the tripping reflex kicks in, rushing your free leg forward. So you just erect a little by bringing your hips forward and your leg will follow.
The lunge is best when it's snappy, it will create that impact we are looking for in running. But you will need that platform to keep your hamstrings save.