The shoulders need to remain over or in front of the bar for as long as possible. This will bring the bar into the hips (or the hips meet the bar) allowing a forceful hip extension up and not forward. A forward hip extension will 'hip slap' the bar pushing the bar away from the body. This makes catching the bar with load near impossible due the projectile of the bar in a looping manner. The bar needs to remain in contact with the body or very close to torso (couple of cm or so) so the projection of the bar is up and slightly backwards. The explosive hip extension coinsides with an explosive shrug and pull from the arms. This cocontraction allows the bar to continue its velocity from the hang position.

A good starting point to learn the snatch is a hip snatch. Take a grip on the bar so that when the bar is held, the bar is at hip height. Flex hips slightly so the shoulders are over the bar. Forcable contract glutes to create a explosive hip extension from a small movement and catch the bar. This position is where the bar should be when the explosive hip extension occurs whether you are snatching from the floor, hang or blocks. The bar needs to be brought into this position.