During the clean and the snatch the bar should always move in a mostly vertical line above your feet. At no point should the bar move in front of your toes nor behind your heels. If you find yourself having trouble keeping the bar in this plane, then you might be using either back extension or hips driving forward towards the bar rather than a jumping movement throughout the second pull (when the bar is explosively moving from just above your knees all the way up to the crease). There is a simple fix to keep the bar moving in a vertical plane over your feet and close to your body……JUMP straight up in a balanced position.
You do not want to think about the second pull in a clean nor a snatch as a powerful hip extension. Keep it simple, jump straight up. Drive your feet against the floor explosively as if you are trying to jump as high as you can. This will result in a complete extension of your hips, knees, and ankles all at the same time, putting the most force into the bar while also maintaining a vertical path of the bar directly over your feet. The objective is to have all of the force applied to the bar in a vertical path rather than applying any forces to the bar in a horizontal plane.
As you transition from first pull to second pull be certain that there is no downward movement of your hips. Shift your weight to the center of your feet and then forcefully jump vertically. Once your feet have left the ground, begin to raise your elbows up in an effort to move yourself under the bar as quickly as possible in order to catch the bar. Remember to keep it simple, push or drive your legs against the floor. This will result in maximal velocity into the bar while keeping the bar moving vertically over your feet.