apenas um palpite, mas talvez seja o modo KRO? Do manual:
Switching between full N key and 6 key rollover In FN-Lock mode pressing the ESC and N key at the same time switches the Keyboard to NKRO mode. Pressing the ESC and 6 key switches the Keyboard to 6KRO mode.
e depois um pouco mais tarde:
Note: NKRO mode is not supported on MACs and prevents some PCs from booting