[Pharo-project] subclassResponsibility, how does it work?

Hi All I'm attempting to subclass a class (Point) and I want some instructions passed to Point to go to MyPoint. Specifically I have Point >>asMyPoint >> self subclassResponsibility. In MyPoint I've got asMyPoint ^x@y. When I execute code, I get an error that the subclass should have executed... How does this actually work? How should I do this? Kind Regards Dusty