What is the difference between stimulation mode and expression mode, and does the BWW Petite Portable Single Electric Breast Pump have both modes?

Yes, the BWW Petite Portable Single Electric Breast Pump has 2 phase expression mode - stimulation mode and expression mode. 

Stimulation mode is a short, quick sucking rhythm to get milk to start flowing, while Expression mode has a longer, slow rhythm for milk-ejection.