It seems that everyone agrees on three kinds of defense/attack timing.
- Go no sen: Like beginner one-step timing. Wait for the attack, block it, and then counterattack. Most people use "go no sen" for this idea.
- Sen no sen: I usually see this label applied to to simultaneous attacks. You wait for the opponent to attack and then take him out without bothering to block. There is a reaction-time to overcome, but this is very easy to do in practice because we force students to make such unrealistic attacks. It is also easy in real life because drunks tend to throw awkward, slow punches with lots of telegraphing. However, this might not be what you meant by "sen no sen."
- Sen sen no sen: I've seen this one under many different labels, but the idea is always the same. You attack when you "sense" that the opponent has decided to attack. In other words, while waiting for your opponent to attack, you decide to attack him instead. Karate teachers surround this idea with all kids of mysticism. In my opinion it is a fraud.
Second, imagine how this would play out in a gunfight. The video cameras in the witness cell phones are running. You are in a tense face-off with an armed desperado. Suddenly you "sense" that he is about to draw his pistol, so you slap leather and drill him through the chitlins before he can move. Consummate skill! Jesse James would be proud.
And then they show the video in court. There you are, and suddenly you draw on a man who has no pistol in his hand and you kill him. What sentence are you going to get from that jury? Cold-blooded murder! Everyone could see it. I think this is a situation we would rather avoid, even with fists instead of pistols.
Third, when sensei practice "sen sen no sen" timing, I think they are fooling themselves. They aren't "sensing" the other person's decision. They are using body language and muscle tension to trigger the other person's attack. This is just "sen no sen" timing without the reaction-time penalty. It is a neat trick, but it isn't "reading the opponent's mind." It is more like writing in the opponent's mind.
So I think "sen sen no sen" is a fraud, and threatens the student's legal standing after the fight. On the other hand it is "spooky" and people will never stop teaching it to gullible students.