Magic of Honmo
Teamwork with a friend becomes essential to a ninja in every scene.
Cases I steal into a case and a mansion withdrawing all at once from a battle with other schools, and to set attack all at once contact between friends is performed frequently.
It is difficult to convey an answer when a friend is in the distance whereas it can convey an answer by words and contact when there is a friend near.
Therefore it is assumed that many ninjas tell a friend a signal by tying up this seal.
