Where I started to get rid of flipping through impact was after watching a Mike Maves (aka Sevam1) video where he talks about asking Moe Norman what the most important part of the swing was and Moe replied 'the pivot.' That got me to thinking and one of the questions I asked myself was 'name a successful golfer who had a poor or weak pivot?' The only person I could think of is *maybe* Corey Pavin. Here's a pic of Pavin at impact.
Not exactly a great pivot, but not a bad pivot either. However, I would say by tour level standards, probably one of the weaker pivots out there as the hands have caught up to BUT HAVE NOT PASSED the zipper.
Here's another pic of Pavin just past impact:
Now the hips are really opening here. It *could* be that Pavin's pivot is such that the hands catch up to the zipper come impact which isn't the greatest pivot ever, but then just after impact he keeps on pivoting and the hands eventually trail behind it.
Again, nobody is teaching how to swing the club like Pavin does and he is one of the shortest hitters and arguably worst ballstrikers in the history of successful tour players. But I still would not consider him to have a poor pivot when you look at the big picture.
Unfortunately, popular instruction is almost infatuated with swing plane and if not that, then the grip and address positions. But the history of golf has seen successful players and even great ball strikers use all different sorts of grips, address positions, swing planes and downswing paths. But the history of golf has yet to see a successful player to have a truly poor pivot (Pavin's pivot may be poor by TOUR standards, but when it comes to golfers in general it's still pretty decent).
Thus, I am convinced that the pivot is the lifeblood of the golf swing and if you want to keep that left wrist flat through impact, you need a proper pivot to do it.