With the advent of the new SoC as part of the Raspberry Pi 4, we have also brought with it a USB 3.0 connection amidst other things. Part of this functionality requires certain high-speed pathways from the processor, and those allow for a standard called PCI Express.
Those of you who are familiar with typical Personal Computers (PC) will be familiar with PCIe, especially if you build your own computer. It's the current standard used for expansion hardware, and allows connections to WiFi, Sound and Graphics cards. Usually they are rated PCIex1, x4, x8, x16, etc. Depending on how many 'lanes' they have to allow data to transfer and even some connections like SATA can be put through them.
Well, people want that on the Raspberry Pi as well, for some reason, and some similar Single Board Computers (SBCs) have them already.
Someone has hacked it in, Tomasz Mloduchowski in fact.
This is definitely not for the faint of heart. Currently to get access to the correct pins on the processor, because every connection is underneath the silicone, it requires that you desolder the USB 3 chip to get the board to bare all:
Which gives access to those sweet, sweet pins
I certainly don't envy the solder skills required to get this setup with the right surface mount components though!
There's certainly a lot of practical work to go to make this sensible in any way, if at all, and there's definitely scope for writing support into the Linux operating system for helping it along, if it is even worthwhile at all!
You can read more details about the great work here: Raspberry Pi 4 B+ - PCI Express