You can picture a pocket of air held in the middle, keeping two sides from meeting. Sometimes people build that pocket on purpose, so that a person can pass from one side to the other in safety. Sometimes the air gets in by itself and simply blocks the way.
One branch
被關住的一段空氣，把兩邊隔開
登入就可以看這一支怎麼連回核心畫面。
sealed room between two spaces · noun
A small room with a door at each end, built between two spaces where the air is not the same. People and things move through it from one side to the other without letting the air rush across.
氣閘室