Coercion includes the application of a threat, intimidation, and/or physical injury, i.e. physical or psychological (or both) threats to achieve a desired outcome against the wishes (and free will) of the victim. It can also be considered as "blackmail."
There can be various examples of coercion and if the defense establishes that the victim committed the act under coercion, the resulting documents can be cancelled and/or stand void. Examples of this include personal will, marriage, professional contracts, etc.
If the defense can establish that the victim committed a crime under coercion and not of his own free will, the victim can be absolved of the crime and let go; while the coercer can be tried for the same crime along with the criminal coercion. This is applicable to all crimes except murder and/or when the threat was of minor injury (among others).
For a better understanding, you may want to go through this study: http://www.manuelsweb.com/ethicspaper.htm