Gear Vendors... Confused

[Nova70]

Someone mentioned this in one of my previous threads and I've been researching them for a week or so. I'm confused on exactly what they are and do. Is it an entire trans or a "bolt-on"? I read there site and I'm confused on exactly what they do as well. I want the sum up of what they say because I do not understand. Are they adding gears to the transmission or is there something else going on? The ratio chart they have says, 1st, 1st over, 2nd, 2nd over, etc. Is the "over" a completely different gear than 1st 2nd 3rd or 4th? Please help me understand this thing...

Thanks,
Nova

Edit- How do they accomplish "gear splitting"? This subject is raking my brain...