What causes the burning feeling you get in your throat and lungs when you exercise beyond your normal capacity?

I have heard on numerous occaisions that it is dangerous to press a weight from behind the head, or pull a resisted bar (e.g. lat pulldown, pullups) behind the head. I have heard a few theories, based on either rotator cuff limitations or the risk of damage to the cervical spine. Anybody know of definite evidence/proof or even with solid reasoning and not just naming a few anatomical structures? Thanks

When training in karate, I notice that at breaks when I am out of breath, the instinct is to bend over, resting hands on thighs.

I'm interested in why this developed, whether this has any advantage to getting your breath back, and, if not, what stance will help me get back to form quickly.

I have attended several different types of exercise class over the years, and I'm always instructed to breathe in through the nose and out through the mouth while exercising. No instructor can tell me why.

Is there a scientific reason for this advice?

Heather Shute, Oxfordshire, UK