Every guide I've read on the subject of combat in this game always seems to neglect the crouch button. Keep in mind, this is an advanced-ish technique because you can (and will) flip yourself over and leave yourself helpless on the ground.

So firstly, the default key for crouch is C. You can duck under opponent's swings, although they will still clip you sometimes. But this is just the defensive way of using crouch. The offensive way to crouch can turn your overhead swing into a 1-shot kill with certain weapons, namely 2-handers.

So there's 2 ways to use the crouch to your advantage, overhead swinging and normal side-to-side swings. The overhead just makes the usual overhead swing stronger. The side-to-side swings, with the use of crouching, can render an opponent's shield useless as you hack their knees off repeatedly. The knee-swipes are weaker but never get blocked. So when you master it, you can reliably kill without worrying about being blocked.

For the overhead swing, you want to hit the crouch key as your weapon reaches the peak of its arc. If you move forward while doing this, you can kill your opponent in one fell swoop.

To attack the knee, you want to hit the crouch key about a quarter of a second before your weapon connects with them. You'll see it slant downwards. It takes a lot of practice. Just watch your weapon as you swing. If you hit crouch too soon, your weapon will drag on the ground, leaving you open. Hit it too late, and you end up giving your opponent a nice hug. Or a headbutt to the stomache, depending on balance. The knee-swipe is more effect when swinging Right-to-Left compared to Left-to-Right.