Basically you snap your fingers while simultaneously instilling your chakra in the opponent's brain, causing them to believe that your snapbwas the reason for one of their limbs to contort and break in inhuman ways. I would use it if I noitced that my opponent tended to use a lot of techniques which requireed handseals, no questions.So.. yeah.
Explain the Genjutsu, and tell me how you will use it in battle. If you have any question, you can ask them as well