The upper part of the circuit represents the scope's probe. (10MΩ, 12pF)
The switch connects/disconnects the 2.2K resistor to the output.
When the 2.2K resistor is disconnected, the scope probe's 12pF capacitance is discharged trough the probe's 10M to ground.
According to the book: "At a high repetition rate a square wave many produce what looks like a flat output." But I'm not sure EC displays what the author meant...
