Push The Knees Back In The Pull?

In the pull of the snatch or clean, you shouldn’t need to actively push your knees back.
If your knees are in the way of the bar, be sure that your starting position places the bar over the balls of the foot, the shoulders above or very slightly in front of the bar, and the knees out to the sides as far as the arms allow.
Actively pushing the knees back tends to cause the legs to straighten too much too soon, tipping the shoulders too far forward over the bar and moving the bar too far back relative to the feet. This forces the knees to re-bend too much as they scoop and slow the bar, disrupts balance and creates rocking on the feet, and reduces the explosiveness of the legs and hips at the top of the pull.
This motion also reduces bar speed in the first pull because it creates less shoulder and bar elevation relative to the hips, requiring more acceleration in the second pull.
If you’re pushing your knees back in search of hamstring tension, just settle down—if you stay over the bar as long as you should, you’ll get that tension as the knees extend at the right time and in the right position and balance to actually use it well.
All you have to do is stand up and maintain approximately the same back angle from a correct starting position—the knees move back by virtue of standing, and in this way, the overall position and balance remain correct.

See these videos also:
Staying Over The Bar
How to Stay Over The Bar

Videos by Hookgrip