A gasket ensures a tightly closed unit and allows for pressure to safely build up in the lower section, where a safety valve provides a necessary release in case this pressure should get too high (with clean filters, that should not happen).
The steam eventually reaches a high enough pressure to gradually force the surrounding boiling water up the funnel through the coffee powder and into the upper chamber (C), where the coffee is collected. Although the "boiler" on a moka pot contains steam at elevated temperature and pressure, the water forced up through the grounds is no hotter than that used in other brewing methods – up to 90 °C, depending on the stage of extraction.
When the lower chamber is almost empty, bubbles of steam mix with the upstreaming water, producing a characteristic gurgling noise. This "strombolian phase" allows a mixture of steam and water to pass through the coffee, which leads to undesirable results, and therefore brewing should be stopped by removing the pot from the stove as soon as this stage is reached.
MAINTENANCE:
Moka pots require periodic replacement of the rubber seal and the filters, and a check that the safety release valve is not blocked. When the rubber seal is new, it might alter the coffee taste, so a couple of "dry runs" can be made, without coffee or with used coffee grounds to "prime" it.
References:
Navarini, L.; Nobile, E.; Pinto, F.; Scheri, A.; Suggi-Liverani, F. "Experimental investigation of steam pressure coffee extraction in a stove-top coffee maker". Applied Thermal Engineering. 29 (5–6): 998–1004. doi:10.1016/j.applthermaleng.2008.05.014. Archived from the original on 4 October 2017.López-Galilea, Isabel; De Peña, M. Paz; Cid, Concepción. "Correlation of Selected Constituents with the Total Antioxidant Capacity of Coffee Beverages: Influence of the Brewing Procedure". Journal of Agricultural and Food Chemistry. 55 (15): 6110–6117. doi:10.1021/jf070779x. PMID 17608497.
