Making swing changes is a process, and it takes time. Swing coach Butch Harmon is trying to get Mickelson to set up with his hips and shoulders more square to the target at address, so he’s less likely to spin them open at impact. That’s why you often see that big pull-hook of Mickelson’s or a weak slice, because his path gets too out-to-in. Mickelson wants to hit a release cut, and the more he’s able to delay his hips and shoulders—especially his shoulders--on the downswing, the easier it is for him to execute that shot. If those shoulders open too quickly, he has to try and save the shot with his hands and that leads to some big misses.
One thing I really like about Butch is that he fixes things in the preswing, before the club starts moving. That’s a good tip for the average golfer. Most people assume their swing is at fault when something goes wrong, instead of taking the time to see if they’re in the correct positions at address. If you’re struggling with a slice or a severe out-to-in path, as Mickelson has on occasion, make sure your shoulders are parallel to your feet, knees and hips at address, and the ball is opposite your lead armpit. (Many amateurs play the ball too far forward, which encourages the shoulders to be open at impact.) Also make sure to place your right hand on the club from underneath the shaft, which promotes a stronger grip.
One final thought for any slicers out there: To keep your shoulders from spinning open, swing your back to the target and keep it facing the target for as long as possible on the downswing. The longer you keep your back to the target, the more likely you are to swing the clubhead on the correct path (inside-out) and release the club.