Hypersmurf said:
What difference does that make?
He could make three attacks; with Cleave he can make four.
The defender sees fighters like that chop down three of his minions each round all the time, but this one can chop down four! Where dfoes the extra attack come from?
However you explain Cleave, you can explain Cleave from an AoO.
-Hyp.
Okay, this is going into the abstract philosophical question of "What does an attack roll represent?"
To me, an attack roll does not always represent a single strike with a weapon, although it can, as in the case of thrown weapons, projectile weapons, and perhaps Spring Attack, Ride-by Attack and Whirlwind Attack.
Rather, an attack roll represents the chance that the attacker will be able to hit and damage the defender over the course of a round. In melee combat, the attacker is assumed to swing his weapon several times. Individual swings may be parried, blocked, dodged, etc., but the attack roll represents the net effect of all the strikes instead of any specific one. (The issue of "What do hit points represent?" is a tangentially related point - in the "hp = vitality points" system, a "hit" that reduces hit points might represent a weapon swing that does not actually injure the defender, but tires him slightly as he dodges or parries the blow).
If a defender drops his defences momentarily, this creates an opening that an attacker can exploit. This is represented by giving the attacker an additional attack roll - an AOO.
So what does Cleave represent in this system? Perhaps the ability to press the attack on two (or more, with Great Cleave) opponents at a time, so that if one can be dispatched easily, the quality of the attack against the other does not suffer (the attacker still gets to make the same number of attack rolls against him).
So, an attacker with Cleave presses the attack on two opponents simultaneously, and one drops his guard momentarily. He gets an AOO against that opponent, and is still able to press the attack against the other. However, since the other defender has not dropped his guard, the attacker gets no additional advantage against him. He continues to press the attack against him, just as he would if the other defender was not there, but that does not translate into an "extra" attack roll.
Of course, you can also explain Cleave in other ways, e.g. an adrenaline surge that increases the attacker's offensive capabaility, that would allow for an "extra" attack roll.