I looked into this awhile back, about breathing only through the aerobika vs in and out. After some research, I learned that the aerobika is designed and best used for breathing in and out.
It creates oscillating positive expiratory pressure (OPEP), which is most effective when you maintain a continuous airflow cycle (inhale and exhale) through the device. Inhaling through the device helps to prepare the airways for the subsequent vibrations and pressure changes during exhalation.

I had hoped that breathing one way would save some cleaning time!