I think the idea of wrist "snap" has too many different interpretations to be all that useful in general. However, I can see why some people would use the term. My best serves are the ones where I really delay the wrist action as long as possible. The later it comes into the swing the faster the racquet comes around. I think that they delaying and loading of the wrist before contact can be a conscious thing. That last minute "snap" into the ball where you release the wrist and accelerate it into contact is what I think is meant by snap, at least by people who are actually using pronation and not so much flexion. The tricky part for me is the timing. I found that it works best if I have finished the snap and have "let go" or relaxed my arm right at contact. It totally gets messed up if I am still trying to actively use muscles at contact. I think you could imagine it as a playground swing. You have to push in harmony with the motion or you end up reducing speed.