Casting classes

[Jonathan Marler via Digitalmars-d-learn]

How do casts work under the hood?  I'm mostly interested in what 
needs to be done in order to cast a class to a subclass.  I'd 
like to know what is being done to determine whether the object 
is a valid instance of the cast type.  If the code is implemented 
in the druntime, a pointer to where it lives would suffice.  
Thanks in advance for any help.