Hosinsul is commonly translated as `self defense`, but is not quite right. All Taekwon Do techniques could be used for self defense, so TKD itself is self defense. But Hosinsul techniques are used, if the usual Kick and Punch techniques could not be used for some reasons. Maybe you got grabbed, or even strangled, or your lie on the bottom and your opponent sits on top of you. Another reason could be, that kicking or punching is not comparative to the situation. This depends on the law in your country, but normally you have to react adequate on an attack, this is at least an demand of ethics. Overreacting is an crime itself, but could be tolerated in some limits, depending on law.
During Hosinsul, the students learn how to defend against grappling and strangling, and how to use leverage to throw an opponent. The students also learn an important, but often neglected skill in TKD academies: the art of falling.