The wrists can bend in any of three directions at the top of the swing. Call it "cocking", "hinging" or "bending" if you wish - to me, it's all the same basic idea. If you think a flat left wrist is the only position to be in at the top of the swing, take a look at Ben Hogan. Then take a look at Dustin Johnson. Total opposites, and neither uses a flat left wrist at the top of the swing. Any one of the three cocking/hinging positions can work. Find out what's best for you.