Kawarimi is a ninja art revolving around misdirection and the split-second timing of a switch between a 'body' and a location, or between two or more bodies. Usually involving the ninja and anything that can be mistaken for them, (such as a dummy or a mannequin), but not always. As many other ninja myths, it's the subject of fiction and exaggeration, as no real-life executions have ever been recorded.
In Shinobido Edit
Nagi is shown using it during cutscenes as an escape tactic, leaving behind a purple mist. As this is the only magical ability she seems to possess, and it leaves a trace of purple, it can be theorised to have a similar origin to Shu's powers.