Purpose of touching tongue to roof of mouth during meditation

While researching forms of meditation involving concentration specific to the Tibetan school of Buddhism, the meditation instruction tells one to touch their tongue to the roof of their mouth when meditating. Recently, I have been reading a book on Taoism and energy flow, which also instructs the same thing yet for the purpose of completing an energy pathway. **My question:** What is the purpose of touching the tongue to the roof of the mouth when meditating (in concentrated meditation in Tibetan Buddhism)? ________________________________ I would also welcome some answers/comments about other schools of Buddhism related to this.