It is because of the importance of the tongue's vertical motion that may explain why infants with tongue tie or posterior tongue tie have breastfeeding difficulties. In this situation, the tongue is tethered to the floor of the mouth preventing the tongue from moving upward and by surgically correcting, breastfeeding difficulties can be solved easily.
Watch video on breastfeeding problems due to infant factors.
Watch the video I made regarding breastfeeding ultrasound (below).
Frenulotomy for breastfeeding infants with ankyloglossia: effect on milk removal and sucking mechanism as imaged by ultrasound. Pediatrics. 2008 Jul;122(1):e188-94. doi: 10.1542/peds.2007-2553. Epub 2008 Jun 23.