What is the “two-drive trick” that can read Amiga disks on a PC?

Standard belief is that the floppy disk controllers used on PCs are incapable of reading Amiga floppy disks, due to the PC disk controller expecting a very specific low-level format of disks, whilst the Amiga disk controller is much more flexible and choosing a low-level format which is incompatible but can fit more data onto a disk. This has been discussed in a few questions on this site (put amiga floppy into the search box to see them).

However there's references to a "two-drive trick" which allows a PC with two floppy drives to successfully read an Amiga floppy disk and generate an ADF image file.

I found adfread-1.1 which implements the two-drive trick. Its instructions are pretty simple:

- Install fdrawcmd.sys from http://simonowen.com/fdrawcmd/
- Insert your Amiga disk in A:
- Insert the PC-formatted disk in B:
- From a command prompt, run:  adfread.exe newimage.adf

There's no information about how the "two-drive trick" works. Source code is included but largely uncommented so I'm mystified as to how this is supposed to work.

How does the two-drive trick work? Why does it require two drives, and how does having two drives get over the fundamental incompatibility of Amiga disks and PC floppy disk controllers?