Have you ever lifted something heavy or benched pressed and had air flow? When I bench, I always try to breath but I still tense up my body and squeeze my lungs/abdomen/sphincter/etc... I dont think complete cessation of air flow is necessary to call something a valsalva maneuver. I can still let air out while performing a maneuver, I just cant breath normally or in a relaxed way. Also, the airflow can be choppy with full closures interweaving with slight airflow. When I block, a lot of times i try to relax and air does flow, but my vocal chords are still far too tense to create sound.