Yoga Relaxation Techniques

what really happen when you hold your breath very/too long?

i mean, physically, what happens on our body? is there any good and bad thing happen? i heard, in yoga, there are technique for Relaxation about breathing.

your muscles don’t work without oxygen. blood takes oxygen to your muscles and brain so they work. without it you faint and die. by breathing deeply you calm your body. you get the oxygen to the muscles you are working during yoga and it helps to keep you under control without straining yourself.

Thai Massage Techniques 2, Relaxation Yoga Therapy for Pain & Stress Jen Hilman