How did a full plate armour function on the battlefield?

Was he an tank, immune to blow and such! his only risk was the lucky blow to a joint in the armour or what?

OR

Was armour more to take lesser blows, but the person inside it would still be wounded, by a good hard blow, not mortaly wounded, but he could survive the battle to "LIVE, HEAL AND FIGHT ANOTHER DAY", where a unarmored person would be killed or crippled.

OR

what?

Well, it's not quite so black and white. But a 15th century knight, in full plate, was incredibly well protected. Against a sword, plate is very effective, with the main vulnerabilities being the openings in the joints. Period fencing texts show many grappling techniques for armored combat, as it isn't that easy to simply thrust into a joint of a moving target that's fighting back. Much more practical to get in close to put the person in a lock or throw them to the ground, then finish them off.

Against weapons such as poleaxes and such, the armor is still very effective, but much less so. Heavy impact can really ring a guy's bell underneath a helmet, and proper strikes can still dislocate joints or twist something the wrong way. A large spike can pierce the armor joints much as what I was saying with the sword, and there is also the possibility of piercing the armor itself with a proper strike in the ideal circumstances.

I recall a story (I wish I could remember where or the exact details) of an armored knight who was on a horse. He was surrounded by foot soldiers (I seem to recall them being peasants, though I don't remember) who used pole arms to yank him off the horse, and killed him as soon as he fell. Even though he was in full armor, being surrounded and outnumbered, and knocked to the ground, the armor didn't do much to protect anymore, so it still comes down quite a bit to the person inside to make sure they get out of the way.

A few other miscellaneous details I could add:
--A full suit of plate armor (including the arming garmets worn underneath) was not good at dissipating body heat. It probably wasn't a problem as long as a knight was riding a horse and probably only using the muscles of his arm(s). But on foot, where the wearer has to use the big muscles in his lower body, heat buildup could be a problem as there is no provision to evaporate sweat, which is the body's cooling mechanism. Quite a few knights died from asphixiation or heat exhaustion without even being wounded.
--And as been stated, even a knight in full plate would be vulnerable to a group of several attackers. Once he is driven to the ground or even just grappled, weapons can be driven in through the gaps. There is a graphic account of a knight being dispatched by having a dagger hammered into each eye slit. Wish I could recall the context.
--By the time full plate reached it's apex, it was being progressively rendered obsolete by gunpowder weapons which, unlike the longbow, could be used effectively by relatively untrained conscripts.
--Also, at the later stages of it's development, plate was as much (if not more so) a vehicle for advertising the wearer's status that it was for actual protection. Rich engraving, gilding, and fanciful constructions covered plate armor. Unfortunately, at the same time plate was being seriously challenged by gunpowder arms, the steel used to make plate was being made SOFTER to make it easier to decorate!

Brian M