Istvan is right on.
try doing a heayy lunge and allowing your knee to move forward over your toe... then do the same lunge never allowing your shin to move forward of vertical. The latter will be far stronger and easier to recover from.
Part of the split jerk is being able to stabilize in that receiving position, part of which is preventing yourself from sliding forward in the split. With the shin vertical this is easy - it's a strong position. Once the shin passes forward of vertical, you very quickly lose that ability to push back off that foot.