here's the topic: your character is composed of body points, which are taken off when you suffer critical hits, and you have not much of body points; you can improve your total by proving yourself a true sports champion.
you can use a substitute of body points called hit points whenever you enter combat mode. you can enter combat mode volontarily. combat mode is a strenuous activity, so you cannot normally always stay in combat mode. your AC raises in combat mode, and you gain feats derived from the fighter class as well.
BUT a character taken by surprise under non-combat mode has no defense like those given by combat mode and can be easily defeated. so how can we define combat mode?
it is viewed as a concentrative ability, based on sharpened senses, with a limited duration in rounds like a haste spell (or perhaps constitution rounds), and usable your level times per day, so you won't necessarily enter combat mode in each and every fight; sometimes you let the friends take up the fight.
my main reflexion with the idea of combat mode as a simple spell which empowers you (temporarily) to the class of Fighter is to permit spells of combat for the magician; if we could "recreate" the fighter class under a spell class, that would let us simplify the system enormously!