polymorphic supertype reference exceptions

I was reading folowing lines and I did not understand clearly.
If a method is overridden but you use a polymorphic supertype
reference to refer to the subtype object with the overriding method, the compiler
assumes you are calling the supertype version of the method. If the supertype version
declares a checked exception, but the overriding subtype method does not, the compiler
still thinks you are calling a method that declares an exception
Any ideas, resources,sample code,links, highly appreciated. thanks in advance