Jousting Rules of the Order of the Banda, ca. 1330

Each knight runs four courses
A broken lance beats no breaks
Two breaks beat one
A broken lance that unhelms counts as two
Unhorsing counts as two lances, even if the lance doesn't break
Knocking a knight out of the saddle beats knocking down horse and man "because in this case the fault was the horse's and not the rider's"
Lances only count as broken if they are broken by striking with the point. i.e. if the lance misses and the jouster breaks the haft on his opponent's body, it counts for nothing.
If a knight drops his lance while charging, his opponent should raise his lance and not hit him.
There will be four judges, two assigned to each team.

Under these rules, although they intend to give extra credit for unhelming, the rules-lawyering in Froissart with lightly laced helmets makes sense. If you are unhelmed but your opponent's lance doesn't break, he gets no credit.

But the tactic is only effective on the jousting field under particular rules: it would be quite perilous on the battlefield. Later rules put a stop to such capers. Those for the Valladolid jousts of 1434 specify that one who is unhelmed may joust no more that day.

Any martial art that aspires to provide useful training for actual combat experiences a constant struggle between the rules-lawyers and the rule-makers.

Fallows, Noel. 2010. Jousting in Medieval and Renaissance Iberia. Woodbridge, Suffolk: Boydell Press. pp. 209-210