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In the split jerk, we’re looking for a solid trunk when we receive the bar—the spine neutral and everything braced forcefully. It’s common to instead see the lower back hyperextended, or abrupt hinging at the thoracolumbar junction. This is not necessarily a dealbreaker in the jerk—plenty of massive jerks have been and will continue to be done like this by even the best lifters in the world. But for the sake of minimizing potential problems and maximizing longevity in the sport, it’s a good idea to try to avoid it when we can. 

Let’s cover the possible causes for the back to be hyperextended in the split and what to do about each one.
 
 
Limited Overhead Mobility
 
Limited shoulder and/or thoracic mobility is the most obvious problem. If the arms can’t move into position overhead freely, the lower back will extend more to rotate the whole upper body farther back to bring the bar into a position that can be balanced.
 
The solution is equally obvious: improve your mobility. See the following videos for help with shoulder and thoracic spine mobility:
   
 
 
Weak Abs / Poor Bracing
 
As in most lifts or positions in weightlifting, if your trunk isn’t braced adequately forcefully, and/or your abs aren’t strong enough, you lower back is likely to wind up hyperextended. For a number of reasons (such as a common lack of trunk control and poor hip and overhead mobility), it tends to be the loose piece in the chain for people, and consequently the center of failures.
 
Trunk stability work should be included every training day to address all aspects: flexion, lateral flexion, rotation, isometric, and anti-rotation for the abs (not necessarily every one of those every training session), and back extension and isometric stability.
 
The trunk should be pressurized and braced properly for all lifts, even the lightest warm-ups—like anything else, the more you do it, the better you get at it, and the stronger it will become. See the following video on bracing, and you can read more in this article.
 
 
 
Back Leg Too Straight
 
In a proper split position, the back knee should be slightly bent. The straighter the back leg is, the more the hip flexors of that side are pulling the pelvis into anterior rotation, which is extending the lower back. Adequate hip flexor flexibility and a properly braced trunk will combat this, of course, but there’s no reason to not work on splitting properly.
 
 
 
Pushing or Driving Forward
 
Often related to the previous is a forward drive or push of the arms into the split. This tends to put a lifter into a split position that’s too far over the front foot and extends the back leg too much (encouraging that excessive anterior pelvic rotation). Additionally, a lifter will tend to naturally try to compensate for the forward imbalance and bar position by essentially bending backward, further extending the lower back.
   
 
 
Not Driving High Enough
 
Finally, if a lifter fails to drive and elevate the bar adequately, they’ll sometimes be able to still sneak into a split by essentially wedging themselves under the bar. Along with soft elbows, this typically involves the lower back hyperextending, both as a way to “shorten” the body, and as a result of an excessively long split with the back leg too straight.
 
 
 
Use this as a guide to diagnose the source of your lumbar hyperextension in the split jerk, and start working on the fixes. Some will be quick and simple, and others, like improving mobility, will take you quite a while. However, in all cases, make forceful trunk bracing a priority—this is always critical in the jerk, and will mitigate the negative effects of any of these problems until you can resolve them.