This kendo terminology, "sen" is an important part of our kendo life, especially in the early stage. Thus, we must really understand what "sen" means...
If you just say this word "sen" to a Japanese, they think that you are trying to tell them something about 1000 (the number).
However, in kendo, we use "sen" as in "sen wo toru", which means "to anticipate (what your opponent will do)".
It is important to know this word because 1. senseis use this word all the time, 2. we want to achieve this level.
There are "Sen no Sen", "Sen Sen no Sen" and "Go no Sen".
"Sen no Sen" is that you know what your opponent is going to do so you will strike before your opponent moves.
"Sen sen no sen" is a kind of "sen no sen". However, in this case you sense what your opponent will do. This involves your intuition.
You can probably do "sen no sen" by looking at the movement of your opponent carefully; i.e., how your opponent looks at you, how he/she moves his/her shinai and so on.
You can tell what he/she is going to do by just looking at your opponent.
"Sen sen no sen" is more mysterious. It is totally beyond my level.
Maybe I have done it once or twice without thinking or even noticing what I was doing.
You just sense what your opponent will do so even before your opponent will do anything, you can strike your opponent.
"Go no sen" is that you know what your opponent will strike, so you let them strike and then you strike.
For example, you know that your opponent will strike your men so you let him/her strike your men. Then you block it and strike his/her do.
Some people use "Tai no sen" as well. Apparently this is kind of the same as sen. You know what your opponent will do.
This time your strike and your opponent's strike are executed at about the same time. However, your strike will be a bit faster than your opponent's so that you can win.
We must be careful though. "Sen wo Toru" is very important and it means you strike before your opponent. But we should not be just hitting whenever we want.
First of all, we want to be in control of the situation we are in. We cannot let shikai mess with our mind. Then we can take "sen" over or anticipate our opponent.
In any "sen" you want to do, you must be in control. This is the key. And also it is not easy...