Over the years, I have received training in a variety of martial arts, and along the way, I have managed to pick up a few useful features from each one. Occasionally, though, some of those things come back to bite me in the ass.
This time around, the bug/feature in question stems from fencing. When engaged in that stylistic form of swordplay, you and your opponent both are wearing head-encompassing masks generally made out of fine, but heavy, mesh – the sport uses blunted, dulled blades, but the whole, "Ow, you just poked out my eye," thing tends to be a drag whether the tip is pointy or not. In any case, you cannot very well see your opponents’ eyes, nor can they see yours.
So what do you watch when squared off against them? Good luck trying to watch the sword; the damned things move fast enough to leave very healthy welts through long-sleeve shirts if you are not being careful. The person’s hands are likewise somewhat pointless, given that they are generally obscured by the bell or guard of the sword handle. So there are two schools of thought I was taught: one says to focus on the shoulder facing you, and the other says to focus on the person’s chest.
In both cases, you are watching a location that does not move very much, but when it does move, you are probably going to have to do something about it in the very near future. Personally, I found that watching the chest – a spot somewhere around where the manubrium and actual body of the sternum meet up – makes it much harder for the person to "fake" you out, while simultaneously allowing my peripheral to keep track of those shoulders as necessary, especially when I defocused a bit.
Regardless, that is just me, and I am far from being anywhere near "skilled" with any kind of sword. However, all of that training eventually carried over into karate when I was a teenager; after all, when you are sparring, you are geared up in a light-weight Red Man Suit, and with the headguard in place, you cannot always get an accurate read on the other person’s eyes. So watch their chest.
Now that I am older and think about such things from time to time, it turns out that "watch their chest" is not always the best of tactics, especially not when paired off doing one- and two-step sparring in "plain clothes" against a female about a foot shorter than you.