It's a combination of knowing when and how fast bacteria multiply, how you can stop bacteria from multiplying, and how you can kill the bacteria that did multiply. Above 40° bacteria will double every 20 minutes. So if you have 5 bacteria on a piece of chicken, at the end of hour 1 you would have 40. At the end of hour 2 you would have 300.
When the surface of whole muscle meats reaches 140° (or the internal temp of ground meat) multiplication of bacteria stops... but it's still alive and well. Now is when the chart that
daveomak
posted above comes into play because it takes into account temperature and time expressed as a result of a logarithm often referred to as a log reduction. The higher of a log number, the higher percentage of pathogens are killed. A 5-log reduction means that 99.999% of bacteria will be killed. A 7-log number means that 99.99999% of bacteria will be killed.
Using a chicken wing as the example, when the surface temp reaches 150°, it will take 5 minutes at that temperature to kill 99.99999% of the surface bacteria. If you allow the internal temp of a chicken wing to reach 150°, and could hold that temperature for 5 minutes, the wing can be eaten because 99.99999% of the bacteria will be dead. The USDA does not keep log reduction numbers a secret, but they do like to publish safe numbers because they are easy to remember. This is why the USDA says to cook chicken to 165°, and if you look back at the chart it says 0 minutes of holding time is needed before eating. The problem is that 150° chicken, held for 5 minutes has a soft texture. But 156° chicken held at least 1 min
So by cooking internal temp to 170 it will be safe am I right