I have to disagree with everyone who says the top players are snapping their wrists intentionally. I think it is a consequence of the swing mechanics.
You can't prove intention or not with still pictures as Toly has provided however cool they look. The wrist does flex yes but pictures cannot tell you if it was intentional or not.
The wrist can be made to flex from many factors: slowing the rotation of the core or shoulders, slowing the arm, shifting your weight at the precise moment, changing the angle of arc of attack, pulling back with your body or some body part (same as weight shift I suppose), altering the intended contact point during the swing, changing tension in the grip or forearm, putting your wrist a specific location and angle during the setup in the takeback also determines when/if the wrist will snap.
When I play I make my wrist / grip as loose as possible. When I hold my racquet during the unit turn and takeback I only put pressure on 2 or 3 of my fingers only to enforce this looseness, kind of like how you hold a pencil that you are twirling. It's that loose.
Then I set my racquet and wrist/arm angles at the end of the takeback depending on how much wrist snap and pronation i want to come out naturally. If I want a lot of snap i put my wrist and arm like so... if I want more pronation w/ less snap I put it like this.... then I just push off and rotate and depending on my setup the racquet and wrist will do what I intended (during setup phase) and will snap and pronate according to my preset plan. I don't have to manipulate during the swing at all. sometimes I will when i miscalc or i get an unexpected hop or i change my mind etc but the setup will determine everything. I believe the pros are doing the same.
Thats my opinion anyway.
Yonex VCore 100s - SW 351 6pts HL
Tour Bite / Luxilon 4G 16 @ 51lbs