When receiving the bar overhead to complete a successful snatch, the bar must be pulled to a "Critical Pulling Height Distance". If the bar does not reach this height, a successful lift is impossible.
Now, when a lifter does snatch pulls in his/ her training, it behooves that lifter to try and achieve that "Critical Pulling Height Distance" on each rep of their pulls. Thus to achieve this height consistently, a height gauge is recommended. And to determine the height that the gauge is set, measure the distance from the platform to the top of the sleeve of the bar, while the lifter is in a full overhead squat ( See the above illustration).
Now, the two above illustrations shows one of my protégés, Bristol Marunde. The top photo shows him in a full overhead squat position. The height of the bar in this position is his "minimum" or "critical pulling height distance". If the upward travel of the bar after a pull stops before this height, the bar can NOT be received at the overhead squat position. Thus the lift will fail! This is the law of inertia, and the only way that this could change is if an addition upward force were applied to the bar later after the momentum of the pull stops, and this is unlikely.
Bristol's "Critical Pulling Height Distance" is about 4" above the base of his Sternum. To be more precise, the distance measured from the floor in that overhead squat, or you could say, where the cross bar of the height gauge rests when he stands relaxed beside it. However, when looking at the second picture of Bristol extending on a snatch pull, the bar appears to only have reached the bottom of his sternum, not the 4" above as was originally measured. The bar is still pulled to that "Critical Pulling Height Distance", it only appears lower because he is fully extended up on his toes.
You will see from the above drawing that the Sternum is situated above the bottom of the center of the rib cage above where the abdominal cavity would be situated.
The two above photos show two stages of a snatch, NOT A SNATCH PULL. The first photo on the left shows Bristol in a "full finished extension". The second picture on the right shows him dropping under the bar to receive it in an overhead squat, but you will also notice the bar still continued to elevate higher even after that full finished extension was achieved. This is all because of the momentum of the bar from its initial acceleration.
Now, when practicing snatch pulls, I recommend pulls to the height shown in the picture on the right, NOT the height in the picture on the left. Let the momentum of the bar follow-through. Don't practice cutting your pulls short.