Re: Boxing "loose"
How long have you been training? I find the majority of novices (like 95%) struggle with being tense and simply need time to loosen up. I agree with your coach to some degree. It's not so much the practice you get from sparring, but the confidence you get from it that allows you relax and loosen up. I wouldn't try to analyze it too much from the perspective of stance or posture, but focus on frame of mind. Novices tend to fight very anxiously, and therefore tense up way too much when blocking/defending, and load up too much on punches. Seasoned amateurs and pros put a lot less effort in defense and punches - because their technique is better. Observe how pros effortlessly duck and roll with punches with minimal movement and facial expression (e.g. James Toney). I wouldn't suggest you copy their technique to a tee, but observe how little they jump & shuffle around, how their movements are fluid and try to copy that aspect of how pros fight.