can someone help me to resolve this problem?

[iscmos]

The attachment is the circuit and simulation results.
It is somehow a RC filter, but the resistance is determined by two switchs and capacitor, but I don't understand the operation even I know the results, so could someone give me some clues? Thanks

 

[FvM]

Switched capacitor circuits are handled in any modern analog IC text book, e. g. Razavi, Design of Analog CMOS IC. The basic theory can be found even at wikipedia.