Complex sentence: Part way through the second half of the cycle there is another valve position change to vent the gas in the first cylinder back into the ambient atmosphere, keeping the concentration of oxygen in the pressure equalizing reservoir from falling below about 90%.
Split into simpler sentences: Part way through the second half of the cycle there is another valve position change to vent the gas in the first cylinder to vent outside. That keeps the concentration of oxygen in the pressure equalizing reservoir from falling below about 90%.