I can't explain in detail but i can tell u that reverse macro is when you add a reverse adapter ring to your SLR/DSLR mount, it allows you to screw in your lens head in first where the rear element of your lens is now facing front. This way, the magnification is larger and can take objects at very close distance, some even closer than 1:1 ratio.
Do not that these reverse adapter rings come with various thread sizes to fit different lens thread diameters and only manual focusing is allowed, unless you get those with AF such as those from NovaFlex (i think it's spelt this way) for around $429 but it's an overkill. The usual adapter ring costs around $45.
Why didn't I think of a rubber lens hood? I was holding the lens and trying to shield the gap at the same time.. I think it shouldn't matter for reverse mounting, if you use a wide enough lens, it should be able to cover, but you will get some nasty aberrations at the corners.
I see you're using a G lens? Otherwise you'll jsut need to open up the aperture using the ring and it will remain open. I only had a Sigma 18-125 when I had to shoot that grain of rice! Had to do so many thing with my hands, I had to get a colleague to press the shutter for me.