You may have heard of these expressions:
sen no sen
sen sen no sen
Basically, "go no sen" means to respond to an attack -- to counterattack. Someone attacks you and in response, you block (or do some other technique). First the attack and then the response. There is a delay.
"Sen no sen" means a simultaneous response. Someone attacks and you counter at the same time. There is no gap.
"Sen sen no sen" basically means a preemptive movement or technique. The opponent is about to attack and you prevent it. This may seem to violate the maxim that "Karate ni sente nashi" -- "there is no first attack in Karate," but really it does not. It all depends on how you look at it.
Someone walks up to you and without warning pushes you. You catch your balance just in time to see a right punch coming. In response, you block with your left hand. This is go no sen.
You know what's coming next. Simultaneously, you both throw punches (his left and your right), but you take the inside line and punch him in the face. This is sen no sen.
In this position, the attacker intends (hopes) to kick with his left foot. Before he can begin his movement, you prevent it by kicking him him the groin and taking him down to the ground. Your kick preempted his. This is sen sen no sen.
Viewed as part of the ebb and flow of the confrontation, the "sens" begin to make more sense.
Actually, we use many techniques that would be classified as sen sen no sen. For example, when we enter and strike to the face, we also trap or pin the opponent's leg. He has not even thought of kicking yet, but you have prevented it.
When Sensei Chosei Motobu visited my dojo with his friend and assistant Takeji Inaba, he emphasized the importance of multiple simultanous movements. For example, in one of the Motobu Choki Juni Hon Kumite forms that he teaches, you block up with your right hand, check a punch with your left hand and bump the attacker with you right knee. The block is go no sen, the check may be sen no sen or sen sen no sen, and the bump (or trap) would be sen sen no sen as it prevents a leg movement. All of these happen at the same time, overwhelming the attacker's senses and making it extremely difficult for him to respond effectively. Taking advantage of this short circuit, you strike down with your right elbow. As a counterattack, this would be go no sen but it followed a simulaneous flurry of "sens."
Sorry for making "sen" plural, but I hope you get my idea.
You are walking along a dimly lit street and see someone lurking in the shadows ahead. You cross the street and walk around the threat. Wouldn't this be sen sen no sen? You have preempted the possibility of the attack by avoiding contact.
A person calls you an idiot. Rather than lose you temper and react, you remain calm. You diffuse the conflict. Sen sen no sen again?
In most of our kata, the first movement is a block and the second movement is a strike of some sort. You block and them counterattack. This seems like go no sen. But what if the block breaks the attacker's arm or disables it? Then it is preemptive as well as defensive. Thus, even a sequence that appears to be go no sen, can actually be sen sen no sen, especially as we advance in our training.
Charles C. Goodin