Critical as this is the place where you will be supporting all the weight that you want to be snatching.
For example, if you want to be snatching 250lbs, you need to and you better be able to support 250obs overhead.
As always, head and eyes straight ahead, big chest tight back, core right, elbows externally rotated (rotated to the ceiling) and you want to stack the bones - you want the barbell (from a vertical perspective) over the wrist, over the elbow, over the shoulder, over the hips, over the knees, over the ankles.
As you can see, bringing the barbell back just an inch - when I provide a little bit of tension, right away the athlete knows this is not strong.
When the bar is brought an inch forward of proper position - the same holds true.
But when the bar is behind the head, and is in a "stacked" position - I provide a lot of force and the athlete supports it with seemingly no effort.
So - get your overhead position correct and lift big weights! =D