The forensic doctor Qiu Xing's abilities were at a level 2.5 in forensic pathology, which was just enough for him to understand what Jiang Yuan was doing.
Put simply, it was about inferring the murderer based on the state of the victim's injuries.
One could estimate the assailant's strength and the position from which force was applied based on the depth and angle of the wound left by a knife.
When a hammer struck the skull, the shattered bones could be used to infer the assailant's height, dominance of hand, as well as their strength, among other factors.
The human body is an excellent force recipient, and moreover, sufficiently researched. For instance, when dealing with the same hammer blow, if it struck a wall, investigators might have to look up the data to estimate the strength, but with the human body, experienced forensic doctors could recite the data from memory.
If the injury left a mark on a bone, there would be even more factors to speculate on.