The concept of "The Center Line' comes up alot in dueling (and in martial arts in general of course).
Generally it's considered a good idea to either have control of it, or get off of it.
The center line could be thought of as the line dividing the left side from the right side. It can also be thought of as one of the most dangerous places to be, because being on it or crossing it is the easiest place for your opponent to hit you.
It's a strange, dynamic entity however, this 'Center Line', and is constantly shifting between you and your opponent depending on your relative orientation. YOU have a center line, and so does your opponent so who is on who's center line is continuously changing.
An added complication is that your weapon can be on the center line, and bear little relation to your own center, or your opponent's and still cut.
So what and where is it? And more importantly how do you learn to defend it/control it and not be caught on it undefended?
In general during training we practice avoiding taking hits, which is as it should be, but if you want to understand the center line, I recommend actually taking hits as an occasional practice.
If this was an empty hand game, to do this you'd be eating alot of shots. Luckily in a weapon art you can use the hand instead - a legitimate target in it's own right - and practice learning to 'see' without the hazard of micro concussions.
FYI - Short weapons are easier than long ones (points if you know why) and obviously padded are better unless you have VERY good control.
Practice striking at your opponents body - cutting/slicing works better than poking. Have your partner tag your hand whenever it comes into range, either before the strike or after (with control) whenever they think they can hit it (they can practice accuracy and reading range).
First do it standing still, then moving around. Remember one ONLY goes for the body, the other ONLY goes for the hand.
Please note - if you, as the 'body striker', are trying to avoid being hand tagged you won't learn anything. If you, as the hand tagger are avoiding getting hit or covering too much, the striker will have no targets to go for that make sense, so the exercise again loses meaning.
Start easy .... there's plenty of time to get tricky later ... and yes, there are add ups ....
So ... Where/when does the hand get hit? What are the parameters of this space? How is your opponent's center aligned to it? How is yours? Where is their weapon relative to them/you at the moment of contact?
Try it. It's a very useful thing to know.