In Stimulation mode, the white LED lights will be blinking, and you will also notice that the suction cycle speed is higher. In Expression mode, the white LED lights will be constant, and the suction cycle speed will be slower. The LEDs will automatically dim when you are not using the controls, so you may need to touch one of the buttons (pause/play or intensity level) to see the LEDs turn back on.
You can easily switch between Stimulation and Expression modes in the app or by pressing the ‘drop’ button on the hub.