How can digestion rates give information to forensic scientists about the time of death?

The digestive organ has a certain transit time that needed for the food to arrive at that organs. The stomach will keep food for 2-3 hours, so if there is food found on the stomach, the time of death should be less than 2-3 hours.

Transit time for small intestine is 3-4 hours. If the food found in the small intestine, it should be more than 2-3 hours but less than 5-7 hours.
The approximation can be influenced by many things though.