PartOfLink ConceptNode: finger ConceptNode: hand
means the same thing as:
EvaluationLink PredicateNode: PartOf ListLink ConceptNode: finger ConceptNode: hand
At this time, the AtomSpace does NOT define a PartOfLink. However, like all 'sugary' link types, this type might be defined in the future, as part of some subsystem that finds it handy an convenient for some particular calculation or another.
PartOfLink is not handled directly by PLN. However, it may be useful as a shorthand for a PartOf predicate, as it's so often used.