Jan,
As the mother slides her finger farther and farther back, slowly, to eventually
reach the juncture of the hard and soft palate, the baby gets used to having
something (and particularily something that is flesh) reach back much farther
than he's been used to.  It trains them to take the nipple in past the bubble
palate and back where it belongs.  I've had some real success with this method.
Yaffa Stark