The reason for the term is historical and is based on the way class inheritance diagrams have traditionally been drawn: with the root at the top of the page, growing downward. (Of course, you can draw your diagrams any way you find helpful.) The inheritance diagram for Instrument.cpp is then:

Casting from derived to base moves up on the inheritance diagram, so it’s commonly referred to as upcasting. Upcasting is always safe because you’re going from a more specific type to a more general type – the only thing that can occur to the class interface is that it can lose member functions, not gain them. This is why the compiler allows upcasting without any explicit casts or other special notation.