I'll expand, as the question is a bit basic. I have looked at a lot of combat, fought occasionally in full plate, watched some of the full contact fighting that is now happening and someone in full armour seems to be pretty much untouchable beyond being battered to the floor and then presumably 'shived' in the gaps till they die. Especially with swords, regardless of how sharp they may be, they seem particularly redundant against plate.
Are there any historical sources that refer to actual combats and techniques and how their use defeated their opponents? I don't mean training manuals as they are showing techniques but obviously are not real accounts of combat.
I'd hate to think William Marshall just rode around bludgeoning everyone he fought into a coma then knifed them up.
^(Edit: William is a poor example as he is from the wrong period... so replace him with some other knight of quality from the right period and you're good to go.)
As you mention in your edit, knights in Marshall’s day wore chainmail, not full plate, but they were still effectively invincible. Importantly, their wealth protected them as much as their armor did, since if they were captured alive they would be ransomed. But I think the most common way to for a knight to die in battle would be to either fall or be battered from their horse and then trampled. Crossbows were also able to penetrate their armor. Richard the Lionheart was killed from an infected crossbow wound. The ability for crossbows to penetrate a knight’s armor prompted calls for them to be banned. As armor got more technologically advanced, falling from your horse and either being bludgeoned or stabbed in a weak spot by ground troops or trampled by warhorses would have continued to be the biggest danger.
It’s no coincidence that knights spent many hundreds of hours practicing being bashed and lanced while trying to stay on their horses. A lot depended on being good at that.