The character gives the subject a one-word command, which the subject obeys to the best of his or her ability. A very reasonable command causes the subject to suffer a penalty on the saving throw (from -1 to -4, at the DM's discretion). A command of "Die" causes the subject to fake death. A command of "Suicide" fails because "suicide" is generally used as a noun, not as a command.
At the start of each commanded creature's action after the first, it gets another Will save to attempt to break free from the spell.