Unless heat can quickly spread through the entire bottom of a pan, "hot" and "cold" spots will develop. The hot spots will be directly over the places where the heat source comes in contact with the pan. Thus, if the gas burner is starfish-shaped, or if the configuration of the electric coil is a spiral, the hot spots will follow those patterns.
The problem of frying or braising in a pan that has hot and cold spots is that you cannot cook the food properly — unless you do nothing else but constantly and thoroughly stir the contents (and when braising, you could not do that even if you so desired). The food over the hot spots will overcook. Or, if you lower the heat to prevent scorching, the food will take longer to cook or there will probably be insufficient heat to cook the other portions of the food.
If you discover that your pots have hot spots and you do not wish to replace the equipment, you can minimize the defect by using a heat diffuser or by using a low heat setting.
When cooking food in a generous quantity of boiling or simmering water, you need not worry so much about the negative effects of hot and cold spots on the bottom of your pan. By the time the heat reaches the food, the cooking medium (water) will have more or less equalized the two temperature extremes. The same principle holds true for steaming.
The speed at which heat can travel through a pan's bottom is a function of how well it conducts heat. Conductivity varies mainly according to the type of metal as well as the thickness and finish of the metal.
