I'm a bit confused.
This thread seems to have proposed, through different ideas, two starkly different candidates for what one means by "wrist snap," perhaps both being correct.
1. The "wrist snap" is the quick change in position of the hand from laid-back to nearly in line with the forearm (the hand becoming less extended), which can be achieved without any forearm pronation
2. The "wrist snap" is a deliberate pronation of the forearm shortly before contact and continues to complete a WW wiper-like forehand follow-through, possibly with some flexion as Anatoliy showed with Monfils.
Now, is the main question whether either of these "snaps" requires any immediate/last-second input from the player (as opposed to simply a passive result of proper prep/takeback/grip/etc.)?
From personal experience, I can say that for me, number 1 happens without any additional last-minute tweaks of the wrist.
EDIT: Best to never again say the words "wrist" and "snap" together ever again; everyone will be better off