PIR Motion Sensor Switch Wiring Diagram 

There are other safe and appropriate ways to wire these same devices, depending on what you want to accomplish with the system. Most people prefer to have the line power going to the switch FIRST because when the switch is OFF everything beyond the switch is DEAD. (sensor and light.) This wiring diagram allows the switch to determine when the system runs. This might make sense if you have a light you want to run when you are away from home but NOT run if you are home. A light for a dog yard might be an example, if you tend to take the pooch with you but still want some sort of night security deterrent when you are away. But if for some reason, you want the light to be controlled by the PIR sensor, thus coming on whenever the PIR sensor tells it to, the PIR sensor could be powered FIRST, then power would run to the light. The switch would be LAST in the sequence and wired between the light and the power source on the return leg of the current's journey. This would mean the PIR and Light always function as conditions dictate BUT the switch would serve as a KILL SWITCH to be able to quickly shut down the entire system. That is somewhat like the way wiring at gas pumps work so that the normal power flow is ALWAYS ON unless the kill switch is activated to shut down the pumps.

I'm not sure I understand the diagram correctly, but doesn't mean that PIR works only when the switch is turned on? isn't the PIR supposed to have constant live wire?

Light abd ire self contained with 3 wires out

Haha. You must have missed something in this diagram. This diagram makes absolutely no sense. Only the neutral is properly connected here, while the live and output should be the other way round.