Purpose of step?

Last evening, I was at a Praying Mantis school for the first time. At the beginning of the class, they all did a step I've never seen before and was wondering what it was and its purpose. I didn't get a chance to speak with anyone after class to ask.

As they walked - about a 1/2 step to 3/4 step, they lightly slapped into the rear of the ankle with the instep of the other foot, i.e., left step, right instep slap; right step, left instep slap.

No, need for anyone to answer this. I found the info on the Southern forum.