A
cadaveric spasm can sometimes occur when a
person is instantly
killed by
violent means (such as by a
gunshot to the head or a
stab to the heart). It doesn't happen when a person is killed by being
burned to
death. The key thing is that the person's
skeletal muscles have to be working at the moment of death. In the spasm, the person's muscles seize up; the condition can be (and sometimes is) mistaken for
rigor mortis.
This condition can be useful to crime scene investigators because whatever the person was holding at the moment they were killed will be literally clutched in a death grip; such items can include hanks of hair or rags of clothing from their murderer.