One part of self-defence is not to make yourself a target for attacks. This can be as simple as not stand out in a crowd by looking or behaving in a such way that you get more attention than the others around you. By blending into the crowd you will less likely be a chosen target. In this way you will hide yourself and become invisible to persons that is looking for victims.
By concealing your feelings you do not give away the opportunity for someone to manipulate you and use your weaknesses. You hide your feelings.
If you don’t show what you know or don’t know you will always have the upper hand and the possibility to use the element of surprise. In a fight for example you do not take a kamae that reveal what style you are trained in. You hide what you know.
In Bujinkan we have this saying…
身を忍び Mi wo shinobi
心を忍び Kokoro wo shinobi
死を忍ぶ Shiki wo shinobu
Hide yourself, hide your heart, hide your knowledge.