Normally air flow is what causes the air flow sensor plate to go down. The mixture is enrichened due to the increased airflow. If you depress the air flow sensor plate, the engine doesn't "know" it was your finger that did it, it assumes that the airflow actually did increase, the amount dependent on how far you pushed the plate. It incorrectly enriches the mixture, enriches it enough to stall the engine. The duty cycle changing is a reflection on the enrichment that is being done by the EHA (Electro Hydraulic Actuator).