In order to execute a proper vertical lift in your backswing, your bottom hand must be actively engaged in keeping the club from laying off when you take the club back and up. It's a natural lift, so you don't need to take the club away low or over-extend with your arms. Just allow the club to lift vertically while making sure you stay out of the sacred burial ground.
Your right hand (left hand for lefties) really has to go to work to keep the club vertical. It's very easy to get lazy, which will cause the club to lay off and get flat in the backswing. This essentially turns your light club into a heavy club, which can make for an extremely difficult process of getting the club back to square at impact. This kills your clubhead speed, which is critical for power and distance.
Watch the video above and I'll give you some tips on how to keep the club vertical using your bottom hand.
Keep it vertical!