Break Vector
All this works, in case I didn't say earlier, because of the extensive use of vectors in the BBC operating system. All operating system calls, and various other functions, are vectored, meaning that their addresses are indirected through other locations, or vectors.
By changing those vectors to point to custom code, it's possible to replace whole parts of the operating system, or for people like me with much more modest ambitions, call some custom code and then continue to the original operating system code.