The term active refers to the mechanical ventilators forced expiratory system. In a HFV-A scenario, the mechanical ventilator uses pressure to apply an inspiratory breath and then applies an opposite pressure to force an expiratory breath. In high-frequency oscillatory ventilation (sometimes abbreviated HFOV) the oscillation bellow and piston force positive pressure in and apply negative pressure to force an expiration.
The term passive refers to the mechanical ventilators non-forced expiratory system. In a HFV-P scenario, the mechanical ventilator uses pressure to apply an inspiratory breath and then simply returns to atmospheric pressure to allow for a passive expiration. This is seen in High-Frequency Jet Ventilation, sometimes abbreviated HFJV.