A vacuum coffee maker operates as a siphon, where heating and cooling the lower vessel changes the vapor pressure of water in the lower, first pushing the water up into the upper vessel, then allowing the water to fall back down into the lower vessel. Concretely, the principle of a vacuum coffee maker is to heat water in the lower vessel of the brewer until expansion forces the contents through a narrow tube into an upper vessel containing coffee grounds (as water temperature increases, dense liquid water increasingly converts to less dense water vapor gas, which takes up more space and thus increases pressure); when the water reaches and exceeds the boiling point (so the vapor pressure equals and then exceeds atmospheric pressure), the (water vapor) pressure in the lower vessel exceeds the (atmospheric) pressure in the top vessel and water is pushed up the siphon tube into the upper vessel. During brewing, a small amount of water and sufficient water vapor remain in the lower vessel and are kept hot enough so the pressure will support the column of water in the siphon. When enough time has elapsed that the coffee has finished brewing, the heat is removed and the pressure in the bottom vessel drops, so the force of gravity (acting on the water) and atmospheric pressure (pressing on the upper vessel) push the water down into the lower vessel, through a strainer and away from the grounds, ending brewing. The coffee can then be decanted from the lower chamber; the device must usually be taken apart to pour out the coffee.
The iconic Moka pot coffee maker functions on the same principle but the water is forced up from the bottom chamber through a third middle chamber containing the coffee grounds to the top chamber which has an air gap to prevent the brewed coffee from returning downwards. The prepared coffee is then poured off from the top.