The rule of thirds refers to having three different areas of heat to control your cooking. If you’ve a gas barbecue with three burners, this is easy to achieve. With a charcoal barbecue, you need a little more effort.
If you fill your grate along it’s whole length with charcoal. When you’re cooking you’ve no cool areas to place your food. When fat starts to drip out of the food onto the charcoal, it burns and flares-up, blackening the bottom of your food. You can juggle your food around but pretty soon you’ll have flare-ups all over your grill and you’ll have to remove the food from the grill. What you really need is an area on your grill that has no charcoal underneath.
To apply the rule of thirds, imagine your grate divided into three sections width ways. In the first third, lets say the left of your grate, put two layers of charcoal. In the middle put a single layer and leave the right third charcoal free. This effectively gives you an area of high heat, an area of moderate heat and an area of low heat. The area of the grill above the charcoal free grate will have no direct heat but will get some indirect heat from the other two sections.
The rule of thirds has two advantages over the uniform heat of a grate with evenly distributed charcoal.
If your grill isn’t large enough to use the rule of thirds consider having a uniform level of charcoal over two thirds of the grate and leave the last third charcoal free. This will give you and area to place food away from flare-ups. Or to keep cooked food warm.