Are you able to lock your wrist without tensing any other muscles in your arm?
There can be a sort of "light and lively" spring-like tension in the wrist that imobilizes it while allowing a full and fluid motion in the rest of the arm. Watch any accomplished violinist to see what I am talking about. The control and finessing of the violin stroke is so far beyond what any cueist uses in play that it's not fair to compare them. However, someone who is very familiar with both motions (as I am) can see that they are very similar in essence. The wrist in never "locked" per se, but it is without a doubt capable of being either totally imobilized or set into motion at the discretion of the player.