If you can use gestures or a graphic, this is very effective.
Point to the pump: "When you push this pump..."
Gesture a rising motion with your hand: "...a platform at the bottom of the bottle rises up."
Gesture pushing something out: "It pushes the cream up and out through the nozzle."
Make a "stop" gesture: "The key is: the space left behind is immediately filled by the platform. No air is allowed in. Ever."