when sprinting my left wrist flops allowing my hand to turn backwards after it passes my hip on the down swing. How do I stop this from happening?
Do you need to stop it from happening? I’ve seen quite a few flopping wrists in world class runners.
My coach told me that it is causing my arm to not go back as far, therefore, shortening my stride length.
If it restricts shoulder range or hand elevation (frontside mechanics) and you want to change the action, you personally need to develop the awareness during running.
Recommend you hold a small stick in that hand as a proprioceptive device and it may be easier to “see” out the corner of your eye and self-correct. Some guys find they can achieve that by wearing a wristband, but try the stick first and then reduce the oddity factor by progressing to the wristband.
Another reason for eliminating the hand flop is that when the wrist is rotated so the hand can flop downward it may have the undesirable effect of “switching off” (for want of a better description) the shoulder muscles on that side. Not sure of the truth in that observation, but it was something a once prominent coach told me.
Take it with a grain of salt.
Marlies Gohr of the former East Germany - three times European Champion in women’s 100m - was a most notable example of a top sprinter with limp wrists (I could give others but he’s retired now in all his pomp and glory 
) but it didn’t adversely affect the range of shoulder movement (or hand elevation) she and her coach(es) were after, so I suppose if it ain’t broke, don’t fix it.
rubin williams of tennessee runs will flopping wrists as well. there is a pretty good pic of that on trackshark right now