To keep it simple you have a device that increases pressure through a closed circuit to push air into the lungs, when its time to exhale it drops pressure (although is still positive pressure i.e PEEP) that is weaker than the natural recoil/elasticity of the chest wall combined with gravity in a supine patient and allows exhalation.
Agree. I'd add that my comment above about exhalation being passive is true in adult ICU practice. In high frequency oscillatory ventilation (HFOV) exhalation is active (a piston actively creates a negative pressure in the breathing circuit). I understand this is still used quite a bit in neonates (though I don't do NICU, so not an expert) but has gone out of favour in adults in a big way due to the results of trials like OSCILLATE (https://www.nejm.org/doi/full/10.1056/NEJMoa1215554). Probably still used in some centres as a rescue therapy, but I haven't used it in years (we'd use VV ECMO for that purpose).
