Much of your brain is devoted to your tongue. It is a huge muscle, constantly moving, that has to keep out of the way of your teeth, help you swallow and avoid choking you. It’s covered with densely packed touch receptors that constantly update the mental map of the shape of your mouth. And your tongue is connected to the brain’s language centres so it often moves to partly form word shapes as you think.
All this sends a huge stream of data to your brain. Sticking your tongue out or biting it, reduces its movement and cuts down on this torrent, which leaves more brain-power available to concentrate.