Found it...
The threshold pin is initially at ground because C1 is charging through R1 + R2. The output pin is at +Vcc.
As soon as the potential of the threshold pin reaches 2/3 of Vcc , the output pin switches to the off state and the discharge pin goes to ground potential.
This causes the capacitor C1 to discharge through R2.
When the trigger pin goes below 1/3 of Vcc , the output pin goes back to the on state and the discharge pin goes to a floating (disconnected) state.
The entire cycle repeats indefinitely.