^^^a "drop/gravity step" is where the foot nearest to the intended direction of travel moves underneath the body (as opposed to stepping away from the body), thus creating a dynamic imbalance toward the direction of travel.
A "jab step" is where the foot nearest the intended direction of travel steps out, away from the body.
If Nadal is turning his foot as he lands his split (I haven't watched the video), then he is performing a "pivot step"
Hope that makes sense?
Do you expect me to run forward and get it in one bounce when I am hitting? - Sureshs 2014