Japan has crazy patents. OM has probably patented this as a way of emulating NDs. The limitation on OM is that the shutter must be rather slow. You cannot do live ND at say a shutter speed of 800.
Sigma has this on the FP cameras, but better. Instead of being for emulating NDs, it is simply a lower ISO. This means dramatically good shadow performance. The problem of course is that the FPs have no IBIS, so you have to bring along a tripod.
But on OM with MFT sensors the advantage of low ISO emulation for noise would be massive. I've tested this inside where shutter was quite slow and it worked really well, virtually noiseless images. You can of course emulate Sigma-style low ISO on any camera with burst mode and averaging frames (termed AHDR), or by doing conventional HDR (which your camera probably has a mode for anyway). You could even use frame averaging to emulate an ND, but it'd be a huge pain and I'd rather just carry a filter or two.
