The tongue "undulates" in a wave from front to back (imagine ocean waves continually rolling onto a beach). When a baby is nursing, she expresses milk with her tongue's movement, combined with some suction and, of course, her mother's let-down reflex. When you pump, you only have suction + letdown. Until breast pump technology radically changes to mimic the natural movement of a baby's tongue, pumping will...well...suck.
Here's an ultrasound image overlaid on an animation of infant tongue movement.
Note: This last video mentioned tongue-tie. Tongue-tie can cause breastfeeding difficulties, but be sure to read Nancy Mohrbacher's article Tongue and Lip Ties: Root Causes or Red Herrings? Too often, she notes, a mother will have breastfeeding issues and attribute them to tongue-tie, when other culprits might be causing the nipple pain or baby's poor weight gain. She notes:
When mothers focus only on tongue or lip tie, other issues may be overlooked and problems can continue for weeks or months....Is tongue- or lip-tie revision the right thing to do for some breastfeeding mothers and babies? No question! But because tongue tie is the root cause of the problem for a minority of babies, it is a terrible place for most mothers to start. When nipple pain or weight-gain issues occur, a much better starting point is to contact someone who can help adjust baby’s latch and evaluate baby’s feeding pattern.