i think its just an adrenaline boost when your body is put into a high state of...lets say, emergancy, like riding down a hill at high speeds. you're using all your muscles and all your brain power to keep control of the situation. you're not thinking about anything else but making it down that hill, so when you feel, mentaly and physicaly exited, or hightened, like this, your adrenaline kicks in, and flows through your whole body, even into your brain, and thats why you feel more responsive and more powerful and feel you have better reflexes, becasue in a cense...you do, because of all the adrenaline.
but this "boost" only last like a couple of seconds and then your body is back to normal because its gotten used to the situation you are in (berraling down a hill at 40-50kmph). you only get this "boost" when your body and mind undergo quick, split second, extreme increase in activity, so to quickly compensate for this hightened need of function, your body releases adrenaline so you can safly deal with the sudden situation, that you wouldnt have been able to deal with on a normal basis. and then the adrenaline goes away because the body cannot sustain such high activity for too long.
im not a doctor or anything, i've just seen some stuff on discovery channal and what not, and i threw in some of my opinion based on my personal experiences