A more thorough explanation:

The useless-gesture exception is a rule in criminal procedure that allows police to enter a premises without announcing their purpose when it is clear that those inside already know why they are there. This exception is a deviation from the knock-and-announce rule.

The knock-and-announce rule requires police to knock on the door and announce their identity, authority, and purpose before entering a residence to execute an arrest or search warrant. This rule is meant to protect the privacy and property rights of individuals.

For example, if police are executing a search warrant for drugs and they see people inside the house flushing drugs down the toilet, they can enter without announcing themselves because it is clear that the occupants are aware of the police's purpose.

Another use of the word "knock" in legal contexts is in reference to a knock-for-knock agreement. This is an arrangement between insurers where each will pay the claim of its insured without claiming against the other party's insurance.

Lastly, "knock in" refers to the act of a law clerk or bailiff rapping on the courtroom door to announce the entry of one or more judges.