A petechial hemorrhage is a small red pin-point mark that is a sign of asphyxiation due to external airway obstruction. Usually, petechial hemorrhaging is caused by blood leaking out of ruptured capillaries in the eyes due to pressure in the head, which puts pressure on veins. However, petechial hemorrhaging can be seen in any part of the body where capillaries or veins may be ruptured due to pressure or straining, leaking blood into the skin.
Positional asphyxia is asphyxiation caused when someone's bodily position obstructs their airways or doesn't allow them to breathe properly.
In the case of petechial hemorrhaging occurring in the neck and chest in positional asphyxia, this means that enough pressure was put on the chest and neck to rupture the capillaries and/or veins there during asphyxiation. With the blood unable to move into other areas of the body, the force of the blood in the capillaries ruptures them, allowing blood to leak out and create the red pinpoint marks by which we can identify petechial hemorrhaging.