In martial arts, the concept of “Sen-te (先手)” means “initiatives” taken to time one’s response during the time of combat. This strategy comes from the book of Five Rings by one of the greatest warriors of Japan Miyamoto Musashi. 
In the Sen strategy, there are 3 types mentioned below: –

    1.   Go No Sen 後の先 (defense and react with Offense)
2.  Sen No Sen 先の先 (defense with the attack at same time)
3.  Sen, Sen No Sen先先の先: (Anticipate your opponent’s attack and counterattack at the same time)

Go No Sen 後の先 (Défense and React with Offense).
In this strategy, you wait and let the opponent attack, and in response you evade, block, and time your counterattack immediately.

Sen No Sen 先の先 (Défense with Attack- same time).
Anticipate your opponent’s attack and evade, block, or parry his attack with simultaneously your counterattack. It will be all at the same time. This levels you higher than the timing of Go No Sen.

Sen, Sen No Sen先先の先: (Anticipate your opponent’s attack and counterattack before his attack).
Anticipate your opponent’s attack, and before he completes his attack counterattack him with your technique. This is the most superior initiative requiring a great sense of timing, alertness, focus, and reaction speed.

Karate practitioners or Sensei can practice this strategy in class teaching Novice students the importance of Go No Sense (Defense and Offense). Students who are proficient in Go No Sen can shift to the
intermediate level of Sen No Sen (Defense with Offense), Attack, and Defense simultaneously.

Advanced students who have trained in Go No Sen, and Sen no Sen and have achieved proficiency can train and prepare with the Sen Sen No Sen strategy as will elevate their levels and is the correct way of training.  This Sen strategy will be a visible indication and progress of one’s timing and reaction abilities.