Accounts of deeds of arms in the 15th century frequently mention thrusting against visors. It is very difficult to achieve significant penetration of a steel or iron steel plate 1.5 mm or more in thickness with even a two handed thrust, and most visors seem to have been at least that thick. What were these attackers trying to accomplish?
Visors present a number of vulnerabilities. While eyeslots were usually a very small target on a medieval helmet, an accurate or lucky thrust could penetrate them, with unpleasant consequences. Chastelain reports that when Jacques Lalaing fought Diego de Guzman at Valladolid, Lalaing “turned the lower point of his axe, and struck three blows, one after the other, within the eyeslots of Diego, in this way: he wounded him in three places in the face…the first stroke on the left eyebrow, the next on the bottom of the forehead on the right side, and the third beneath the right eye….”
Given the number of blows that penetrated Guzman’s eyeslot without hitting an eye, I suspect that Lalaing was deliberately choosing angles of attack that were likely to produce a bloody wound, but unlikely to blind his companion.
And having achieved the difficult task of penetrating his opponent's eyeslot, it's possible that his point never left it for the next two blows, instead pulling back only far enough to shift to a different aim point on Guzman's face.
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. 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.”
Finally, the numerous breaths that perforated the visor of the typical helmet intended for foot combat presented another point of vulnerability. In at least one case, the chronicler suggests that a very acute point might actually slip through the breaths far enough to injure the face. De la Marche reports that when Bernard de Bearn, Bastard de Foix fought the Lord of Haubourdin in a continuation of the Pas de la Pelerin of 1446, Bernard bore an axe with a lower spike that was “long and delicate, fashioned so that it might easily enter the holes of a basinet, and long enough to do great damage to the face of his companion”. This seems to have been unusual enough to draw comment. A very slender spike would be vulnerable to breakage. Probably Habourdin had visor holes that were small enough to exclude the typical robust pollaxe thrusting point, but vulnerable to the unusually acute point chosen by Bernard de Bearn. Habourdin seems to have been distinctly miffed by Bernard’s tactic. “When he was advised of the subtlety of the said axe, he said that he didn’t want to make his companion take pains to pierce the visor of his basinet. He quickly had his detached and entirely put aside, so that his face remained entirely uncovered.”
Even if we accept the above example as atypical, perforation of the visor created a point of weakness. De la Marche recorded this exchange between John de Compais and Antoine de Vaudrey, fighting with estocs or thrusting swords at the pas de l’arbre de Charlemagne in 1443 “And finally de Vaudrey pierced the visor of his companion, and when de Compais felt it pierced, he threw his estoc with all his strength at the visor of his companion, and with that stroke they were both similarly taken in the visor. Each champion held the other by the pierced visor, and they lifted their swords so that both of them had their face naked and uncovered, and at that the judge threw down his baton, and had the guards restrain and separate them.”
It seems probable that both champions struck a blow that either glanced across the visor of their opponent until it reached a breath, or hit a breath directly. When it did the weapon point sank into the breath and lodged firmly.
At that point, there are a number of potential outcomes, and few of them favor the party struck. The point lodged in a breath may give an opportunity to lever a visor open. Alternatively, the striker may use the point lodged in his opponent’s helmet as a lever to put his opponent at a disadvantage.
Finally, a powerful thrust to the face could potentially stagger or even stun the target, particularly if it lodged in a breath so that it didn’t glance. Helmets were padded, but on most surviving helmets the padding was relatively thin, and much of the effectiveness of the padding depended on the way it also served as a suspension system. This was more effective against a downward impact than a horizontal blow to the face.
While a visor offered substantial protection, it was still a point of weakness compared to many other parts of the harness. Attacks against the visor were a logical attempt to exploit this vulnerability.