The Highflyer is designed to extend the busboard found inside the A4000. The idea is that you remove the original busboard, insert the Highflyer then insert the original busboard into the highflyer. The highflyer include a case which extended the height of the A4000 case. It also came with an additional power supply for powering the Highflyer. The unusual thing is that it does not extend the number of Zorro slots, but extends the number of ISA slots, and provides an additional video port. Exactly why the ISA slots would be useful is unclear, because they are non-active but perhaps the highflyer was designed for a specific purpose such as a Video Toaster system loaded with Time Base Correctors (these only require power from an ISA slot).

Thanks to Iggy Drougge and Erkan Orhun