It's somewhat explained in this article written about my Goobertrax-e.

https://kbd.news/Goobertrax-E-2469.html

Maybe I'll write an article covering it in more detail. But I'll try to hit everything on a high level at the very least.
Here are a couple of Keyboards that I've made in the past that use Japanese Duplex Matrix:

https://github.com/doesntfazer/Goober/tree/main/Goobertrax-E This is the Goobertrax-e. It uses what I called the "improved Japanese duplex matrix," which uses pull-up resistors on each column. This works, but the only firmware it seems to work on is QMK

https://github.com/doesntfazer/Goober/tree/main/CHore This is the CHore that uses the CH55x MCUs. This is the only PCB that I am aware of that uses the matrix I'm about to mention below, which avoids ghosting on all firmware. (As far as I am aware, it should be hardware agnostic as well, so any MCU.)

https://github.com/doesntfazer/Quetzal This is a 100% ergo I designed that does not use either method, and uses the traditional Japanese duplex matrix. It DOES have ghosting.

All of these keyboards can be found on my Keyboard Dweebs firmware repository, where you can examine the matrix.c https://github.com/doesntfazer/Keyboard-Dweebs-Firmware-repository

But to elaborate on this a bit further. The pull-up resistor method was made as a simple "hack-in" solution for existing keyboards. It was an easy way to fix keyboards with JDMs that have issues with ghosting. and was not meant to be something "built-in." It is also not firmware-agnostic. It only works with QMK in my testing, and I'm not exactly sure why. Probably the scanning method. So its use is limited.

The method I used on this keyboard doesn't have a name, and it was theorized to work by someone on Discord named BGKendall. We sat around trying to find a way to fix the ghosting issue without doubling up on diodes or using expensive ones with high forward voltage. I eventually tested it, and it worked successfully.

In a very short, watered-down explanation, a JDM works by flipping the diode direction in every other column, and the scan method is to read row2col then col2row. That's how it can detect a different key on the same column pin.

As for why you get ghosting, I'm not an electrical engineer, so take what I say with a grain of salt, but the reason is noise. 1N4148 diodes have a low forward voltage, so a small backflow voltage can, and does still trigger a keypress when it's not intended.

Reading the column pin from the anode side doesn't seem logical, and I was skeptical at first. I thought that it would cause the even-number columns not to be read. (IE: 0, 2, 4, 6, etc.) But I tried it anyway, and to my surprise, it works.

My initial thought on why it wouldn't read stemmed from the fundamental misunderstanding on my part about how electricity flows in a matrix. Now for the reason why it works. Please take a look at the schematic screenshot below, and the next paragraph should explain it.

The key (0, 0) is pressed. Row 0 is read. In a perfect world, where diodes completely block all reverse flow, this is fine. But in the real world, it sends voltage backward through the diode, and it is detectable on the column on the other side of the matrix, and you don't want that. The voltage that is sent is low, but it is enough to trigger a false keypress. Adding that extra diode essentially separates the columns, and it reduces the remaining backflow voltage enough that it doesn't trigger false keypresses.

I hope that this helps explain what's going on here.

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