CIM stands for cum in mouth, where you ejaculate into your partner's mouth. It is just one of the many ways in which you can end your session, if your partner is happy to try it. It can be a fantastic way to end, as your partner may wish to finish you off with their mouth, meaning that moving to a new position isn't needed. You can simply enjoy the sensations being given to you by your partner and let things happen more naturally.
