One way to defeat armor was removing or displacing it. The Pas de la Dame Sauvage hosted by Claude de Vauldray in Ghent in 1470 gives one measure of the importance of this mechanism.
There were 16 mounted combats. Each consisted of one exchange of lance attacks, followed by the exchange of sword blows, ending after one or the other party struck 17.
During the lance attacks a grant gardebras was torn away, and a gardebras was torn away or unriveted twice.
During the sword attacks bevors were disabled three times by a damaged rivet or strap so they failed to protect the face, and one gardebras was unriveted.
At the Passo Honroso of 1434, during the 25 lances courses run in the first two challenges, four pieces af armour were torn away: the left arm twice, the right vambrace, and armor for the right hand.
Some helmets can be removed entirely by a skillful lance stroke. Jacques de Lalaing, jousting at Nancy in 1445, unhelmed two of his four opponents. Not only would this leave the victim very vulnerable in a real fight, the helmet could do damage on its way off. When Boucicaut was unhelmed at St. Inglevert in 1390 "the blood flew out of his nose."
Pieces of armor could also be carried away in combat on foot, as when Galiot de Baltasin and Phillipe de Ternant fought with two-handed thrusting swords in 1446. "On the third coming together, Galiot hit the lord de Ternant on the bottom of the right shoulder, and with that blow he pierced the gardebras, and carried it away on the end of his sword."
Edge blows cut also cut or break the straps or laces holding up the pauldron. This would not only expose the shoulder but hamper the affected arm. In the portion of the deed of arms fought at Noseroy in 1519 with two-handed swords, there were many "guardbraces brought down...(avalez)"
Also, positive mechanical catches to keep visors closed seem to have been a relatively late development: I know of no clear examples before around the middle of the 15th century and they took some time to become common. Earlier, visors seem to have depended entirely on the friction of their pivots to keep closed, and one or more vigorous upward thrusts could drive a visor open, leaving the face vulnerable to a following thrust. Monstrelet records that when Poton de Saintrailles fought Lyonnel de Wandonne at Arras in 1423: “watching his opportunity, he closed with Lyonnel and struck him many blows with the point of his axe under the visor of his basinet so that he raised the visor, and the face of Lyonnel was clearly seen.”
Hector de Flavy used a similar tactic against Maillotin de Bours in 1431 "Sir Hector, more than once, raised the vizor of his adversary's helmet by his blows, so that his face was plainly seen, which caused the spectators to believe Sir Hector had the best of the combat. Maillotin, however, without being any way discouraged, soon closed it, by striking it down with the pummel of his sword, and retreating a few paces."
So did John Astley against Philip Boyle, in London in 1442. "And that yere, the last day of…………… save on, there was a batayle in Smythfeld, withinne lystes, aforn the kyng, between the lord Beaufe a Arragonere and John Ashele squyer of the kynges house, a chalange for spere to cast pollex and dagger at the lord aforeseyd in brekynge of his gauntelette and reysyng of his umbrary*, and hadde hym at mischief redy to a popped hym in the face with his dagger, tyl the kyng cried hoo: and there the seid Asshle was mad knight in the feld."