Enrico, it was obvious :)
I meant if anybody knows another "tricky" functions

Occasionally found yet another undocumented function - $zle: 

USER> set x=$zlp("a,b,$c(2)") set y=$zle(x) set z=$zel(x,3) zwrite x,y,z
x=$c(2)_"a"_$c(2)_"b"_$c(6)_"$c(2)"
y=3
z="$c(2)"

Who knows more?

Hi Dmitrii,

Why not Sync from main()? 

Class User.DelayedTest Extends %RegisteredObject
{

ClassMethod Callback(interval As %String) As %Status
{
    Hang interval
    Write "Interval = ", interval, !
    Return $$$OK
}

Method RunWorkers(queue)
{
    #Dim queue as %SYSTEM.WorkMgr
    Set queue = ##class(%SYSTEM.WorkMgr).%New()
    For i = 1:1:5
    {
        Set status = queue.Queue("..Callback", $RANDOM(5) + 1) // Minimal delay is 1 second
        $$$ThrowOnError(status)
    }
}

ClassMethod Main()
{
    #Dim d = ##class(DelayedTest).%New()
    Do d.RunWorkers(.queue)
    Write "This should be printed first",!
    Set status = queue.Sync()
    $$$ThrowOnError(status)
    Write "Exiting...",!
}

}