all breastfeeding mothers and professionals alike know instinctively that kneading, squeezing, pinching, pushing, jostling, etc... on the breast by a nursing baby or child is to elicit stronger or more frequent letdowns. all you have to do is watch kittens or other mammals nursing - or milk a cow or goat by hand! - to figure out how it works.
but lest any doubt remain, i present you w/ a video of testimony straight from a nursing child himself - "it makes it go "whick! whick! whick!" :-)