PEP stands for Positive Expiratory Pressure. Blowing bubbles through the tubing into the water creates pressure in the lungs and airways, which helps keep them open. This helps to move more air in and out of the lungs.
The vibration from the bubbles helps to break up the mucus (phlegm), and the increased air flow helps to move phlegm out of the lungs into the airways. From here, it can be coughed up.
By changing the amount of water in the bottle, you change how much pressure is created in the airways.
There are many reasons why Hydrapep is the leading Bubble PEP device on the Australian market: