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For casts of less than about sixty feet or so, no wrist is needed. It can be
used, but it will introduce faults if not done properly. Simply allowing it
to "break" will always introduce faults, and may cause pain as well.

For simple "pick up and lay down" casts, only rotational power application
is required. ( The rod simply travels through an arc).This is easier with a
locked wrist. For long casts, then the stroke length needs to increase.
There are several ways of doing this.

For maximum power, a "wrist snap", as described above is also required. As
hardly anybody needs to cast 90 feet or more with a #5 wt, there is usually
no call for it under normal fishing conditions.

If you use light gear, and understand the mechanics involved, you can cast
using practically only your wrist. This is however rarely a good idea for
most people. Wrist muscles are weak in comparison to shoulder muscles. This
may also cause you various problems and pain.