Once your milk starts flowing, you can adapt the suction rhythm by pressing any of the three expression settings on the base unit. This button then lights up instead of the stimulation button.
Depending on your own personal comfort, you may wish to use a higher suction mode that provides deeper suction. You can always return to a lower suction mode, or turn off the breast pump whenever you like by pressing the appropriate button.
Note: You do not need to use all suction modes, only use the modes that are comfortable for you.