When you need to concentrate on something, like a crossword puzzle, you are using the hemisphere of the brain also used for processing motor input.
It is pretty funny to see people slow down when they are thinking of something difficult while walking.
This is caused by interference from the two activities fighting for the same bit of brain to process them.
Many biologists believe that by biting your lip or sticking your tongue out, you are suspending motor activity and also keeping your head rigid, to minimise movement, and hence interference.
Large areas of the brain are devoted to control of the tongue and to the receipt of sensation from it.
With the tongue held rigid against the teeth or lips, the activity of those areas is subdued, allowing delicate tasks like threading a needle to proceed with less interference.