[wlfpck]

Think pedal position and the amount the throttle body is open.

With the standard cable throttle, it is all mechanical. If you graph the pedal position versus the amount the throttle body is open, you'll get a linear curve. For instance, pedal fully down is 100% open. Pedal half down is 50% open. (Could actually be different but that's the simplified idea).

Now if you take drive by wire (dbw) and graph it out, for most cars, you'll get curve that fits better to an exponential curve or some polynomial curve.

The pedal has a position sensor which senses the position of the pedal and coverts that position into a specific voltage which is used to determine how much the throttle is open.

This is why there are multitudes of comapnies (Blitz, etc) that sell these throttle controllers. It is something that is easy enough to make that you could actually go to radioshack and build one yourself. You just need to know your circuits. Either way, these throttle controllers basically goes between the voltage from the position sensor and the input to determine the amount the throttle is open to adjust the voltage as needed. The overall result is a more linear behavior.