if a processor has a 'devlopment mode' and 'pins' are ports to a debug 'feature' of the chipset, then plugging in a device (or fob) might allow for a triggering of an alternative access method to the features of the board.
some would call this 'debug mode'.
when doing this kind of thing one might also have a separate processor.
for example you could have multiple processors in one Integrated circuit.
or the board it self could have . . . secret processors . . . if it were designed to be used to cheat.
so many possibilities.
when you have a developer mode you 'pause' the process and run some kind of process of the various devices of that board with the processors in charge, and the OS 'blind' to what is happening (except that usually there are 'watch dogs' that 'ping' outside sources, such as a time server, and 'count cycles', but did those boards have that.)
my point: the architecture of the boards and all their devices, and the debug and development modes of those processors could be modified in a way to allow for wireless insersion of manipulative command sets built into a hacked up version of the development tools, designed to run as a side process on parts of the equipment that are separate from the main process, but with full access to the backplanes and debug modes of all components. Well known protocols can be hacked.
all in all for a dedicated team of clowns it would be an endevour but possible.
however a forensic audit of the whole embedded system of these voting machine . . . would uncover it. But they also hacked the process or litigation. So what they did to the boards? we aren't sure. What they did to the judges: sometimes we get news reports about judges.