Ah yes...one of the secrets indeed.
But, (my personal, undocumented, uncertified and unpublished experience) with the res in place and not serviced (not filled/refilled), my rad levels stay the same.
With the res removed, my rad levels drop.
So, while there is loss from the res during a ride, it does seem able to capture relieved coolant from the rads and put it back INto the rads on cooldown.
So, while the res doesn't seem to do what its name implies..acting as a coolant reservoir, it does accomplish something, and thus, its removal for 'improved air flow' while it may seem logical causes other problems.
sez me.