I should have a video about this later, but for now, a written guide.
Luke Skywalker: Blocking red swings in JK2 is random and inconsistent. Sometimes I block, sometimes it just hits through... why?
Mantracker: Partially true. Did you know there is a 'box' around your saber? A big clunky box, unprecise and doesn't really follow the saber perfectly when you look up and down.
Luke Skywalker: I had no idea. Can you show me what you mean?
Mantracker: In fact, yes. This is a representation of the box, revealed by the game's code.
Luke Skywalker: What? I'm in a game? And why is it so small? It doesn't cover my saber.
Mantracker: Yes, you are. And it's because you are in an old game. Or the developers took shortcuts. I don't really know. Now, look up, then down in red style.
Luke Skywalker: That. Is weird. Well, so this box follows where I look essentially. What's the deal with this box anyway?
Mantracker: This is how it works: If the person swinging at you is hitting this box, you will definitely block. 100% guaranteed. You can recognize this block if your saber gets staggered, and your opponent is unable to do combo attacks.
Luke Skywalker: So if I aim this "box" correctly at his "saber box" I will be safe? What if I miss?
Mantracker: Yes. If you miss, and he hits the front of your body but outside this box, there will be a dice roll.
Luke Skywalker: Ah, sweet randomness. How much are we talking about?
Mantracker: A 20% chance to fail. 80% chance to block. Arguably you could say that you were unlucky if you got hit at first strike.
Luke Skywalker: Wait, so what did you mean about the combo attacks? If he hits outside my blocking box, then what?
Mantracker: Then, provided that he holds attack still, he can continue comboing and perform new red swings until he gets the lucky number (1 or 2) - or until you do the perfect block, with your blocking box.
Luke Skywalker: Are you sure of this?
Mantracker: Yes, absolutely.
Luke Skywalker: Well, at least now I can call people out on their luck with more confidence.
Mantracker: I'm on a horse.
Saberbox be tested with this file: