Actually nipple stimulation has been the only scientifically proven at-home method to help get labour started...
Well, actually there is no tale at all, just simple explanation - stimulating nipples (like baby is feeding), it bring out hormone called oxcitocine (hope I spelled it right), and it couses uterus to squeeze (no, there was another word, sinonime, I forgot it - sorry, English isnt my native language). As Your baby hasnt born yet, it can couse contractions, something like after having orgasm.
its like an old wives tale that nipple stimulation can jump start labor