As the club head is making contact wit the ball, the left side of your body (left knee and foot) should remain firm. Never collapse your legs into the shot!
Many players collapse the left knee though the impact point which is a huge no no as it creates a loss of control over the direction of the ball flight. You will see many professionals, including Peter Lonard and Rod Pampling actually set their left legs straight at the address position to avoid the collapsing effect.
Another thought at the point of impact is to never let your left foot move from the original position from which you began your swing.
Obviously, a left handed golfer would keep the right side “quiet” and right foot still for the impact points…