It wasn't because this rabbit demon had any rules but because it wanted to absorb some of the core soul energy contained in other people's attacks.
Body Tempering realm martial artists actually had very little experience in training their core soul energy, especially before the Visceral Tempering realm. They focused on increasing their inner strength cultivation.
Considering mental strength would become more practical after reaching the Visceral Tempering realm or even later stages.
However, if they didn't train normally, it meant they wouldn't use their mental strength. Every time they cultivated their inner strength and used every move, they were actually using their heart's divine power to drive it.
However, these were all instinctive uses, just like breathing, so no one would notice this.
These rabbit demons had some characteristics of aperture refinement state experts, particularly their proficiency in using mental strength.
