When Spring Crown's class changed to Weapon God, he gained several passive abilities. The most unique one was he could now see literal weak points in his opponents. These weak points were shown in the form of small red dots. If Spring Crown hit these weak points, he could score more damage.
While in battle, these red dots might enlarge due to the enemy's movements or the results of getting attacked. This only happened in a short interval, though. If Spring Crown hit these weak points when the red dots enlarged, he would score a critical hit. Additionally, it would also cause the opponent to stagger.
That was what caused Mammon's swing to swerve and miss hitting Jeanny. Spring Crown's perceptive eyes quickly located the weak points on Mammon's swinging arm. His uncanny control let him strike one of these weak points accurately and with perfect timing when the red dot enlarged.