What difference between @event and wait (event.triggered)?

Hi Dave , I tried Muriali’s example as it is using Non-Blocking event triggering but it gives an Error : " An automatic var. or elem. of a dynamic var. (this) may not be the trigger variable of a non-blocking trigger ".Could u pls explain why this error occurs ?

Only If I make the following changes : static event ev ; Use Non-Blocking Triggering ;
fork … join in ex2 and ex4
I get all 4 displays in the O/P