At your regular stance, everyone is aiming at where your head is. If you bend your knees when they punch, or when you punch, they will still aim at where your head was a split second before. You don't just want to keep your knees bent all the time. You want to vary the height of your head as well as the side to side position all the time.
I think the above comment is pointing out the irony of questioning what your coach says and seeking advice from the internet. Which is a far worse source for any information. Here are reasons why this is not a smart thing to do: 1. Even if many people here are qualified to give advice, they cannot see what your coach sees - he can easily see any weaknesses and bad habits that you have and should have the best insight to try and develop your talent. 2. If you don't listen to your coach, he will tell you to go **** off. Boxing coaches are old-school, they don't want to be ****ed around with and waste their time. He will spend his efforts on other boxers that appreciate his advice. 3. Reading up on different strategies and techniques is a healthy habit. But to blatantly ignore instructions and seek help from other sources will confuse you. Even if your coach's talents are limited, you will be much worse off trying to wing it and ignore clear instructions. Unless your coach is an incompetent moron who knows nothing about boxing, and you're way smarter and more experienced than he is (which is possible, i'm not saying it isn't - however very unlikely), you should probably trust him and follow his advice. Maybe he's nit-picking on a minor detail. Or perhaps he's homed in on a major flaw, you'll never know unless you try it. If you're standing up too straight, your stance won't be stable, your punches will not generate enough power, and yes, you will be a larger target. Like a punching bag with legs.
