Hello,
We reworked completely this rule in version >= 6.2 of the Java analyzer, since an older version is included by default in version SonarQube 8.1, you might face less precise results for this rule if you didn’t update anything.
In any way, I believe the current description of the rule is pretty good, you should find plenty of information there.
If you still have questions or something is not clear, feel free to come back to us here.
Best,
Quentin