Quickly then the man in the green made ready,/ Grabbed up his keen-ground ax to strike Sir Gawain;/ With all the might in his body, he bore it aloft/ And sharply brought it down as if to slay him;/ Had he made it fall with the force he first intended/ He would have stretched out the strongest man on earth./ But Sir Gawain cast a side glance at the ax/ As it glided down to give him his Kingdom Come, / And his shoulders jerked away from the iron a little,/ And the Green Knight caught the handle, holding it back,/ And mocked the prince with many a proud reproof:

What rule in the Code of Chivalry, did Sir Gawain break and cause the Green Knight to mock him?

Thou shalt respect all weaknesses, and shalt constitute thyself the defender of them.
Thou shalt not recoil before thine enemy.
Thou shalt never lie, and shall remain faithful to thy pledged word.
A new love puts to flight an old one.

The rule in the Code of Chivalry that Sir Gawain broke and caused the Green Knight to mock him is "Thou shalt not recoil before thine enemy."

The rule in the Code of Chivalry that Sir Gawain broke and caused the Green Knight to mock him is "Thou shalt not recoil before thine enemy." This rule implies that a knight should not show fear or retreat in the face of their opponent. In the given passage, Sir Gawain flinches and jerks away from the Green Knight's ax, displaying a momentary weakness that goes against this code. The Green Knight mocks him for this, as it goes against the expectation of a chivalrous knight to stand steadfast and fearless in the face of danger.

In this scenario, the rule in the Code of Chivalry that Sir Gawain broke and caused the Green Knight to mock him is "Thou shalt not recoil before thine enemy." Sir Gawain flinched and jerked away from the Green Knight's ax, showing fear and implying a lack of courage in the face of danger.