This is a Shell Extension for Windows that allows to customize its shortcut letter for the File Explorer's contextual menu.
Its main purpose is to detect and select a customized list of programs that the user will pick to open any file with, thus, this will speed opening those files in some scenarios.
For instance, you have a C++ source code file you want to open, but you don't like the Visual Studio to launch on double clicking. So, with this shell extension, you will be able to right click that file and select Fast Open With, which will list a custom list of currently running source code editors (or other programs), as they're already running, opening with them will be faster. 🏃🏻💨
As a programmer, I'm more of a keyboard 🧑🏻💻 person than a mouse 🐁 person, so I rather hit the combo Menu Key ≣ + W while selecting a file, since W is less used in the File Explore's contextual menu list. So hitting those combo (which of course it will be customizable to assign any other letter), this will execute Fast Open With shell extension, which will detect the running programs that matches the custom list of executables, and picking them from that list, opening that file will likely be more comfortable.
If none is running, we can always pick the program we want with.