it looks like the wrist snapping, but it's actually the forearm moving from pronation/neutral on takeback, to suppination as you start to swing forward (the racquet flips back) and then neutral/pronated at contact and follow through...
it's the forearm not the wrist and it is a result of a loose arm and slightly shorter takeback than the WTA. if you do it right you'll feel a slingshot type sesnsation in forearm.
your wrist may extend back at some point, but it does not flex forward.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM