Medela SwingFlex single electric breast pump is infused with patented and research-based 2-Phase Expression Technology. Through research, Medela has learned that there are two distinct phases of how babies breastfeed. The first phase, known as stimulation phase is when the baby first goes to the breast and sucks fast to initiate the milk flow. The second phase of expression occurs after milk flow or let-down begins. During this phase, baby breastfeeds at a slower suck rate for continued milk flow.