Usually a split diaphragm, although you'd only expect that to only affect one carb, unless both are split or failing to seal.
There's usually a hole in the bottom of the slide which connects the venturi in the carb body to the space above the diaphragm, although it may be a drilling elsewhere in the carb body.
This means the area above the diaphragm is at the same pressure as the venturi, while under the diaphragm atmospheric pressure is maintained.
When the throttle butterfly opens and air rusehes through the venturi, the pressure drops below atmospheric and the slide lifts against the spring.
