It’s using Context Variables which makes the rule faster as it is able to use the event that triggered the rule rather than having to query SmartThings and check the status of the motion sensor.
So the equivalent of this would be:
IF Living Room - motion is active
But in that case, the rule would have to reach out to SmartThings to get the status of the motion sensor, so it adds a slight delay. (maybe 0.5-2 seconds).
Since are are dealing with motion and we want the rule to process as fast as possible, James is using a trick where you can use the event that triggered the rule in the IF Comparison. This means the rule can instantly proceed with the comparison since it uses the existing value of the the motion event that triggered the rule (rather than having to wait a moment while it asks SmartThings for the latest state).