How does mouseFocus work?

At the moment I do

startDrag: evt
        WorldState addDeferredUIMessage: [
                self attachAllSubnodes.
                evt hand grabMorph: self.
                evt hand addEventListener: self.
  ].

with

handleListenEvent: anEvent
        anEvent isMouseUp ifTrue: [
                self activeHand removeEventListener: self.
                self detachAllSubnodes ].
        anEvent isMouseMove ifTrue: [
                self moved: anEvent ]

and

moved: anEvent
        self positionMeInParent: anEvent.
        self updateParentLine.
        self nodes do: [ :node | node updateParentLine ]

to make sure that I can update the parentLine morph
when moving this morph. Is there some documentation on
how mouseFocus works?

Stephan