The tuning plates in the BBQ smokers help distribute the hot air from the fire box, and allow the smoker to have an even cooking temperature.
The hot air exits the firebox towards the cooking chambers and travels below the baffle and tuning plates. It is therefore forced to move towards the vertical section and the smokestacks. Now that we have the hot air traveling on a "chamber" we can release it, by sliding open the tuning plates, where it is cooler and heat up that spot.  This way we can tune the smoker for an even temperature.

The photo on the left shows the lid next to the firebox. It shows the baffle (the inclined steel plate) followed by the tuning plates. The center photo shows several spaced tuning plates. The photo on the right shows the angle iron rail where the tuning plates rest upon and the bolts that are used to tighten the plates into place.


These tuning plates are on a rectangular smoker. The photo on the right shows the plates going into the vertical section.