Final answer:
No, detaining a person against their will does not automatically place them under citizen's arrest.
Step-by-step explanation:
A citizen's arrest occurs when a private citizen detains another person whom they suspect has committed a crime. However, the statement 'Once you detain a person against his will, you have placed that person under citizen's arrest' is False. In order for a detainment to be considered a citizen's arrest, certain requirements must be met, including having a reasonable belief that a crime has been committed and having witnessed the crime personally. Additionally, the legality and specifics of citizen's arrest vary between jurisdictions, so it is important to consult local laws.