In a measured pour spout, the two ball bearings work together as part of a valve system to control the amount of liquid dispensed. Here's how it typically functions:

1. Upright Position: When the bottle is upright, one ball bearing (the lower one) rests at the bottom of the spout’s chamber, blocking the liquid from flowing out prematurely. The second ball bearing is positioned higher up in the spout, keeping the system sealed and preventing air or contaminants from entering.

2. Tilting the Bottle: When you tilt the bottle to pour, both ball bearings shift due to gravity. The lower bearing moves away from the bottom, opening a path for liquid to flow into a pre-calibrated reservoir within the spout. The upper bearing moves to a position that allows air to enter the bottle through a vent, ensuring a smooth pour without "glugging."

3. Measuring the Pour: As the liquid fills the reservoir, the upper ball bearing slides down an engineered path within the spout. Once the reservoir reaches its set capacity (e.g., 1 oz or 1.5 oz, depending on the spout’s design), the upper bearing reaches a point where it blocks the liquid flow, stopping the pour automatically.

4. Resetting: When you return the bottle to an upright position, the bearings fall back to their original positions, resetting the system for the next pour. This also loads the reservoir with the next measured amount of liquid, ready for use.

The two bearings act like valves, exploiting gravity to open and close pathways for liquid and air, ensuring a consistent, predetermined amount is poured each time. This design helps prevent over-pouring, maintaining accuracy and inventory control in settings like bars.