High ISO NR is a traditional noise reduction technique that looks for random pixels. Long Exposure Noise Reduction is a dark frame subtraction method and is entirely unrelated.
Regarding dark frame subtraction, here's a sample file that illustrates what we're trying to avoid:
The pinkish stuff along the right hand side of the frame is thermal noise, and it's not supposed to be there. This particular frame is from a D2h at 30 seconds, perhaps more pertinently at about 15F - from my LR record it's clearly apparent that we were at this location for at least two hours before this shot, and I certainly remember being cold.
If I had been using LENR, the second exposure would have been with the shutter closed (that's why putting the lens cap on has no effect!) and the resulting frame should in theory have been all black. But it wouldn't be, since all this pink stuff would have been in it. So the camera can then compute that anything not black shouldn't have been in the previous frame, and it can remove it. Attachment#1 (jpg file)
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!