Each company will have their own instructions for the crew of which this is one example. It is not from the company concerned and shouldn't be used as such:
Normally, the Captain will give the command, "EVACUATE", repeated 2-3 times.
If no Flight Crew member has provided the necessary direction, the purser/CCM should attempt to contact the flight deck by interphone to obtain instructions.
An evacuation must not be initiated while the aircraft is moving. However, any Crew Member may initiate evacuation in cases of extreme/catastrophic emergency.
The obvious question here is: what do you define as extreme/catastrophic?