Analogy: Assume a JVM is a kingdom, Object is a king of the kingdom, and GC is an attacker of the kingdom who tries to kill the king(object).
When King is Strong, GC can not kill him.
When King is Soft, GC attacks him but King rule the kingdom with protection until resource are available.
When King is Weak, GC attacks him but rule the kingdom without protection.
When king is Phantom, GC already killed him but king is available via his soul.

So Phantom ref is basically GC saying "Omaewa mo shindheru" and the object saying "Nani???"