"lengthen his plates"

The phrase occurs in the seneschal of Hainault's challenge of 1402. What does it mean?

My theory is that it refers to the sort of groin protection that was not uncommon from 1370 to 1430, and illustrated here. As the illustration shows, it's possible to ride while wearing this sort of protection. However, experiment with modern reproductions of this kind of armor indicates that it's also somewhat awkward, since the groin protection needs to be folded upwards. The protection of the saddle made this type of protection superfluous for mounted combat, and it was made detachable on some if not all harnesses that had it. The effigy of Alexander Stewart , d. ca. 1406, clearly shows the groin protection attached by straps and buckles. Lengthening the plates, then, could plausibly refer to jousting without the plate groin extension but attaching it for the foot combat.

Basinet visors ca. 1400 came in two different forms. On some the left side of the visor, which would be a target for the lance when mounted men-at-arms passed shield to shield, was not pierced with breathing holes. On others holes pierce both sides of the snout. The second style offered better ventilation and vision for foot combat, which put more demands on both. Also, the more frontal foot combat gave less advantage from leaving one side of the visor snout unpierced. From a safety point of view it made sense to prefer one for the joust and the other for combat on foot, so the seneschal allowed a different visor to be used for the combat on foot.

On the whole, the idea seems to have been to encourage protective equipment that wasn't too thoroughly optimized as either specialized jousting equipment or specialized foot harness, but with some minor concessions to greater safety in each role. However, since the axe was a more dangerous weapon than sword or dagger, the seneschal made no restrictions on the sort of armor worn while fighting against it.