Look at the circuit shown. You notice the extra push-button switch S1. In addition, the circuit has only one LED
As long as the push-button switch S1 is pressed, the multivibrator remains quiet. The LED is out.
By contrast, if you unpress S1, the LED1 starts to blink.
In a real world application, the push-button switch could be replaced by a trip wire or a contact at a door. When the trip wire is broken or the door opened, it is indicated by the flashing light of the LED.