the above is a bootstrapped switch. the capacitor acts as a battery during p2, which is the output phase. during p1 the switch is off and the C is charged to vdd. during p2 the source to gate of the main switch is connected by the capacitor which has a constant voltage in it. hence the switch is much more linear, due to constant Vgs.
could anyone else comment on the above? whether it is right or wrong.