The rule as written:

If this optional rule is in use, a broadside can shoot
even when it is not loaded. In this case, the damage
counters the target takes are halved, rounding
fractions down. The broadside cannot be reloaded in
the same turn it shoots with continuous fire.

When playing with Standard Rules, in
addition to the rules above:
—The raking modifer (if applicable) is added
before halving the damage.

— Before resolving any fire, all players secretly
choose the type of ammunition to use for
each broadside shooting with continuous fire.
The chosen ammunition counters are revealed
simultaneously and fire is then resolved.

— Double shot cannot be used with continuous fire.

When playing with Advanced Rules, in
addition to the Standard Rules listed above:
— A player must plan a Fire Broadside action to
use continuous fire (even if the broadside is not
loaded).
— A player may not plan a Reload Broadside
action on the same broadside he planned a Fire
Broadside action.


There are some drawbacks to it:

GUNNERS LACKING TRAINING
If the Continuous Fire optional rule is in use, the
drawn tokens are one third of the normal amount
(round fractions down), instead of half.
You will also notice that once you start continuous fire, you can't issue a reload action. That means you will have a longer delay to fire a normal broadside after using this rule should you need to. You will also not be doing as much damage each turn, which might not knock out full boxes on the target ship damage track. This could leave them doing more damage over the course of the firefight since they would be filling your boxes at a faster rate. Plus, the more chits you deal your target in one shot, the greater your chances of giving them the higher damage and special damage ones.