The vacuum pump
A passive cup holds what it captured. A powered cup holds what it can keep capturing.
Continuous evacuation
The pump runs while the tool is attached, replacing air that leaks past the seal or through the material. On a genuinely non-porous, clean surface it will cycle rarely. On a marginal surface it will run often — which is itself useful information about the quality of your attachment.
Flow versus vacuum
Two different figures describe a pump: the depth of vacuum it can reach, and the volume of air it can move per unit time. Depth of vacuum sets the maximum force on a perfect surface. Flow determines whether it can maintain that vacuum on a leaky one.
That distinction is exactly why a High Flow variant exists.
Listen to it
Experienced operators use the pump as an instrument. A pump that settles and stays quiet indicates a good seal. One that never settles is telling you the surface is leaking faster than it can cope with.
Questions about this
Why does the pump keep cycling on some surfaces?
Because air is leaking in — through the material if it is porous, or past the seal if the surface is dusty, textured or uneven. The pump is replacing what escapes.
Is a constantly running pump dangerous?
It is a signal, not automatically a hazard. It means the surface is marginal. Re-prepare it, and if the pump still will not settle, treat the surface as unsuitable.