Quantified as "he rolled high on his attack roll - which means the opponent fell for it".
You could also say the opponent suddenly wondered if he left the water running in the bathroom sink, which distracted him at an inopportune moment. The attacker has no actual control and a strike to the leg would be coincidental and not intentional. What makes a melee deadly (and fast) is that both participants aim blows at exposed areas and forcing the opponent to expose those areas is part of the contest.
While it make make sense that a fighter who is aware of what his opponent is trying should formulate a plan to avoid the opponent's maneuvers, sometimes it isn't the case.
I could explain to one of my students (Adam) in detail what moves I planned to use, "I'm going to execute a beat attack, an extension, and thrust to your right shoulder."...and that student would do the same, wrong move to counter it. Repeatedly. I'd gather all the students and invite them to come up with answers but even after hearing the answers, that student would still fall for the move. The other students demonstrated various moves to avoid the maneuver (Disengage was what I was looking for but after the beat attack, any defense of the right shoulder should have parried a basic thrust).
While at a war in Estrella Park near Phoenix, I helped Orion, a student, purchase a new, 43 inch rapier, which would give him an advantage over the standard 37" blades. We joined the lines fighting (it was stupid, but when groups of fencers hit the field, they'd form lines)... and I'd graciously introduce myself and my student then, "Today, I and going to execute a
pris de fer and while I have your blade bound, Orion shall kill you with a single thrust." After killing our opponent, I'd thank them for their generous participation and some would stalk off, muttering. We killed over 20 people before Orion got hungry. A fight isn't just two sides flailing at each other hoping a shot will land. There are techniques a fighter may use to almost guarantee a shot will land. Teamwork makes a huge difference when people are trained. The Romans learned it, and it was later used in the Renaissance in the pike formations that included men armed with swords and bucklers whose job was to dash forward and attack the enemy pike men by getting inside their reach. In that respect, the
tercio was an integrated combined arms unit.
Rolemaster is not a combat simulator, it is an RPG that has a well thought out abstraction of combat.
A combat simulator would be: I can see my opponent wearing a short-sleeved byrnie and a leather gauntlet on his sword hand. If I strike him at a random location, a blade striking iron chainmail would likely do little harm as penetrating armor is pass/fail. I could try to strike an area without armor, such as below the knees, the neck or face, but those are small targets and difficult to reach. The neck and face areas also tend to be the best defended. That leaves the right arm, as the left arm is hidden by his shield. The right arm has the shoulder area covered in chain and the thick gambeson used for padding. The upper to lower arm are bare while the wrist and hand are covered with leather thin enough to make gloves, better than cloth but not enough to be considered armor.
An active weapon arm tend to be a vulnerable target when the owner is unaware that it is being targeted and there are feints to use that will lure the opponent to make attacks that will expose that arm.
In fencing, in epee and rapier, the weapon arm was a favorite target. I'd slowly place my blade into a low position where my opponent would lose sight of it because his own arm would be interposed. I'd flick up and tag my opponent's wrist. In a tournament, I pulled that move three times in a row because my opponent didn't figure it out. It isn't quite the
Coup de Jarnac but it worked.