When switch SWl is positive, the FET is turned on, and has a resistance of about 400 ohm. The input voltage charges up the capacitor through the FET. When SWl is negative, the FET is turned off (pinched off). To get a long storage time, the op amp must have a very low input bias current. For the CA3140, this current is about 10 pico amps.
The rate at which the capacitor will be discharged by this current is based on the equation, C (dv/dt) = i where dv/dt is the rate of change of voltage on the capacitor.