Every bong has a down stem or a stem of some kind, Like in and yang these parts need each other in order to rip yourself a cone or two. the stem offers suction while pulling a bong. pulling the smoke straight from the dab or cone piece. Down stems can be either be a fixed item or detachable. Because of this, they offer an easy bong clean or replacement. Although customising your down strem can end up looking pretty sweet too. Some offer better filtration and unique filters.
The process starts as soon as you start pulling a cone. The water will start to bubble while filtering through the water. leaving from the water into the bong itself and then finally into your lungs to get you high as f***. Coming in many different shapes and sizes while being made with many different materials. Moreover the most common being Heat resisting pyrex glass. In order for the down stem to work, it should offer a narrow design while having an airtight seal between it and the cone piece or banger.