Decisiveness is one of the key factors to success in not only battle, but in life as well. Being decisive means being able to take control of opportunities when they present themselves. In Japanese this idea of seizing the initiative is called sen (先). Timing is super important in being able to seize the advantage and therefore there are many different types of sen. Go no sen or ato no sen (後の先) means to take initiative after the opponent has attacked. It can be thought of as a counter attack. Senken or sometimes called senken no mei (先見の明) means to take initiative in anticipation of the partner's attack. Ken no sen means that during the onset of the bout one seizes the initiative first by setting up the opponent. Tai no sen is to wait until the moment of the attack to seize the initiative as the opponent opens themselves up to attack or some use this to mean to feign before the attack. Sen no sen means to seize the initiative before they seize the initiative. Sensei described sen no sen as, "move before they move." Sen sen no sen in Sensei's terms then means to move before they move to move where the initiative to move comes from an almost sixth sense. One would move before they even knew they were going to move.
Generally a beginner can only employ go no sen where they are waiting to be attacked because they cannot "read" their opponents movement. The more seasoned or intermediate practitioner is better at reading their opponents movements and thus can start to use ken no sen and move first. The expert can read their opponents movement but they also can start to "sense" their opponents intention and can then make use of sen no sen. Only someone with countless hours on the mat and master's level of ability can fluidly use their sixth sense and pull off sen sen no sen.
The "Luck is what happens when preparation meets opportunity" quote attributed to Seneca is apropos to sen. Training requires that students and teachers alike be determined and diligent in their efforts. In order to be successful, one must put in the time so that they can be ready when the time comes and thus seize the day.