Breastfeeding acts as birth control?

Was just informed that breastfeeding can act as a form of birth to control (although it's still recommended to use other forms of BC like condoms). 
I also read it says breastfeeding only works as birth control if you breastfeed exclusively. Why does it only work if the baby solely drinks breast milk? 
Just curious as to the science behind the idea that exclusive breastfeeding  acts as BC