With a threshold device, you have to overcome an initial pressure to open a valve, and that’s the target pressure.
Once the valve is opened, there is no resistance to the airflow anymore.
The difference between resistive devices like The Breather is that the resistance that your muscles are working against is constant throughout the entire breath cycle, so your inspiratory and expiratory muscles are working for the entire 2 to 3 seconds during inhale and during exhale. This puts a more sustained workload on the muscles.
On a study commissioned by Dr. Watts of Texas Christian University, they confirmed considerable muscle activation using sEMG during both inspiration and expiration through The Breather. Texas Christian University Clinical Study (Poster) https://www.pnmedical.com/clinical-trials/hyolaryngealmuscleactivation/. Direct comparisons between both methods have shown that both threshold and resistive devices strengthen the respiratory muscles, but that the curve of activation looks different, confirming this sustained muscle activation with resistive devices such as the Breather.
Read The Evidence Of Resistive Vs Threshold