When the heart stops pumping the blood goes to the lowest points in the body and creates an unnatural coloring of the skin. This phenomenon is called lividity.
This discoloration of the skin after death can offer invaluable clues to the investigators. For instance, if a person dies sitting in a chair or hanging from a noose, the bruising will occur in the dead person’s feet and hands. Or if the person dies lying down, the patterns will appear on the underside of the victim’s legs, arms and back. If the person is lying face down when death occurs, the 
bruising will be found on the front of the body. If the body’s position at the crime scene does not match these discoloration patterns, investigators have forensic evidence to suggest that the body has been moved.
So if a forensic investigator finds a body face down but the discoloration is on the back, he/she knows the killer altered the original crime scene.
Here’s Georgia at work in VULNERABLE, assessing a victim.
Georgia sat cross-legged as she lifted the victim’s cold and badly swollen hand. Though the cuticles had receded, she could see that in life the victim kept them neatly filed and painted with a faint sheen of purple nail polish that still caught the light.
Carefully, Georgia inspected the fingernails, crusted with dirt, searching for any sign that the victim might have scratched her attacker. Knowing the medical examiner would do scrapings under the fingernails, she covered both hands with paper bags. Porous, the paper allowed air to circulate so that moisture didn’t form and destroy any DNA that might be present.
“Let’s hope you scratched the hell out of him. Maybe together, we can put this asshole away.”
When both hands and feet were bagged, she gently rolled the body on its side. Pushing up the shirt, she noted a purplish discoloration darkening the backside of the girl’s legs and arms.
Called lividity, the color change was caused by blood settling or pooling in the body’s lowest point when the heart stopped pumping. Forensic technicians used stippling patterns to determine if the body had been moved or repositioned.
If there’d been discoloration on the front of the body, she’d have known the girl lay face down for a time before being placed on her back. In this case, it ran the back length of the girl’s body. This suggested the girl was positioned here at the time of death.