The plant sends more growth hormones to the apex branches. Bending the plant makes for more top area for the growth hormones to go and hence every node along the bent branch will sprout new colas. Try it, super cool to see happen!
And in some cases topping the plant to slow it down is a good thing. What if she's starting to outgrow her veg tent because the girls in bloom are taking an extra few weeks to finish? What to do? Maybe pump the breaks with a little pruning to make some room AND benefit from extra colas down the road, sounds like a win to me. Oh, and then root the cuttings you just took in your propagation method of choice for more plants down the road!