You need to keep the lungs constantly inflated using low tidal volume to prevent damage to the lungs by the ventilator. I'm sure schpier can provide more specifics.
Hopefully, as suggested by the news media, there seems to be a decrease in the numbers of patients needing mechanical ventilation. Nevertheless, there continues to be a shortage of the devices. A few points from my perspective:
The lungs of patients suffering from Covid19 pneumonia/pneumonitis are “stiff” ( layman term). Compare trying to inflate a nice elastic balloon vs a stiff plastic coke bottle.
Moreover, the diseased lung stiffness ( low pulmonary compliance ) is distributed in a patchy pattern, mostly in the peripheral areas. The stiff, diseased areas, require a higher pressure to inflate than the areas of the lung that are not diseased and which are more effective in gas transfer than the diseased parts.
So attempts to apply a high inflation pressure to the whole lung at one time, will place the less diseased area at risk of bursting - since those areas are elastic and susceptible to overinflation ( a balloon popping ).
That is called barotrauma, a well known, critical complication of mechanical ventilation.
To minimize the risk, modern ventilators control for the highest inflation pressure ( peak inspiratory pressure), the lowest pressure ( end- expiratory pressure) and mean/ average airway pressure.
Additionally, since diseased areas of the lung require more time for gas exchange ( both oxygen-in and carbon dioxide- out ) the timing of inflation/inspiration and deflation/expiration must be controlled very carefully.
There are many terms for all those features and controls leading to many names for different types/ patterns of mechanical ventilation, but the essential considerations are as outlined.
Suffice to say that blindly squeezing a rubber bag and forcing oxygen into a very sick patient with very diseased lungs is an obvious setup for disaster.