Basically, zope.app's path expressions use ITraversable adapters to
traverse through the object graph. This is the only sane thing to do,
really. However, we still want to take care of Zope2's OFS.Traversable
which most promimently allowes the __bobo_traverse__ hook.  (The irony
is that OFS.Traversable uses Zope 3-style *URL* traversal these days;
yes, *URL* traversal, not *object graph* traversal).

Still confused?

Basically, if we encounter something that is OFS.Traversable'ish, use
restrictedTraverse. Use Zope3-style object graph traversal in all other
cases....