In IBM compatible PCs there are usually up to two floppy drives, called A and B. Which one is which is determined by a twist in the data cable: if a drive is connected before the twist (as seen from the floppy controller), it is addressed as drive B, if a drive is connected after the twist, it is addressed as drive A. PCs can only boot from floppy drive A, but not from drive B. So if there are two physically different drives connected (e.g. a 5.25 inch drive and a 3.5 inch drive), you are limited from which type of media you can boot, depending on the way the drives are installed.
The pc-floppy-switch removes this limitation by providing an electronic way to switch drive locations.
In essence, the pc-floppy-switch conditionally adds another twist to the cable, making the original twist seem to disappear:
The device itself uses a 74HCT244 "Octal Buffers and Line Drivers With 3-State Outputs" which internally is organized as two sets of four lines being buffered. This is a perfect match, as there are four control lines involved and two alternatives from which only one can be active at a specific time.
Note: this design connects two outputs directly together, which is only a good idea if there is a mechanism in place that prevents both outputs from being active at the same time. The physical ON/ON toggle switch is such a mechanism.