The runner's mass is concentrated in a small compact area called his center of mass, which is usually located just above the navel in men. All bodies rotate easier at their center of mass. If a force is applied to the center of mass, the body will rotate. This is called torque, which is the product of the amount of force applied and the distance from the center of mass.
Torque = F x d
The same amount of torque can be applied to an object at different distances by changing the amount of force applied to the center of mass. If you are farther out from the center of mass than closer in, less force is required. So tackling a runner low takes less force to tackle him than it would if the runner were tackled at or above his center of mass. If the runner were hit at his center of mass, he will not have any torque, but will be driven in the direction of the tackle.
