The Multiface One was a memory dumper for the ZX Spectrum 48K with a button that generated an NMI to run code in its ROM that could dump the machine state to mass storage (CMT/cassette tape, diskette, etc.). This could later be reloaded, and was often used to pirate games. Wikipedia mentions that, "later revisions contained a switch that effectively 'hid' the device from software."
In It's Behind You, Bob Pape (the programmer of the R-Type port) mentions, on page 65:
It was the same when [Rob Hylands] told me of his way to defeat the Multiface 1, it was so obvious but so impractical that you wouldn't have even given it a second thought had it crossed your mind but Rob had, and he had the ability and sheer nerve to pull it off. Rob put his protection system into play in the only Spectrum game he wrote, Super Wonder Boy (in Monsterland), a multi-load version of the SEGA sequel to their 1986 Wonder Boy game released by Activision in 1989.
To see if the method still worked I recently go hold of a copy of the game to run on a PC emulator, set the emulator up to mimic the presence of a Multiface 1, loaded the game and pressed the key to invoke the Multiface. Doing nothing else I exited the Multiface and returned to the game where, just as it had with the cassette original in 1989, the game tore itself apart for several seconds and then completely crashed. The brilliance of Rob's method was that even if I had saved off the game to cassette it was already tainted and loading it back in would have led to the same result, a totally unplayable game.... No, I'm not going to tell you what the method was, since if you know nothing about Spectrum Z80 coding it won't mean a thing to you and if you do then think of it as a puzzle to try and solve yourself.
So how did the Multiface hide itself from software, and how did Hylands defeat it?