What is in the reservoir bag while on Controlled Ventilation?

I ask this question because I always thought the Bag was kinda "disconnected" from the circuit while on the Vent similar to how the APL valve doesn't do anything.

Obviously I see the bag slightly inflate and deflate during the respiratory cycle so I know it's not totally disconnected, but you can usually slowly squeeze the bag and won't see any changes on your respiratory flow.

However, if you squeeze the bag quickly, you will see some variation on the flow so presumably squeezing the bag did deliver some volume to the patient.

I'm assuming this is exhaled gas in the reservoir bag similar to what happens during spontaneous ventilation? Does that mean whatever is in the bag is BEFORE the exhaled gas goes through the CO2 absorber?