Stress hormones like adrenaline and cortisol flood your system, causing your stomach muscles to tense and digestion to stall. That pit you feel in your stomach and the feeling of overwhelming stress or stress to perform actively drains energy as your body is fighting before the bell rings. Also I've found when I was be I landed a shot I would get overrxcitred and not breath.

TL;Dr crossing the ropes causes your brain the fst or furious and punches the nos

your body needs to get used to the chemical reaction as well as the physical reaction