You release the actuator.
The compressed spring pushes the piston back up to its original position.
This upward movement creates a vacuum (negative pressure) inside the chamber.
The inlet valve opens due to this vacuum.
The vacuum simultaneously draws two things into the chamber:
Liquid from the bottle (via the dip tube).
Air from the outside atmosphere (via a dedicated air inlet channel, often built into the housing or piston).
The chamber is now refilled with a precise ratio of liquid and air, ready for the next press.
This cycle repeats with every pump, ensuring a consistent dose of foam each time.