5 1/4" floppy disk drives had little sensors that would detect a notch in the side of the rectangular disk sleeve (well outside of the round magnetic disk inside). Open notch meant "writeable". Manufacturers would sell "one sided" disks cheaper with missing notch for the backside of the disk to prevent using it. You could use a hole punch to pierce the soft plastic sleeve and make a "writable" hole at the correct spot. The disks inside were identical on both sides, there never was a "one-sided floppy disk", technically. This was during the "C64 and everybody got the games by exchanging floppy disks on the school yard" phase of home computing (ca 1985). Prices for floppy disks mattered a lot back then.

The 5¼" floppy disks consisted of the floppy disk coated in magnetic substrate, encased a plastic envelope. The drive mechanism would only have one read/write head, to read one side of the disk. Disk manufacturers would sell single-sided floppy disks, as well as double-sided floppy disks that you could physically flip over to store more data on the other side. The double-sided floppy disks were a lot more expensive. The only real difference between the two types, though, was that the manufacturers warrantied that the second side would work; to save production costs, the disks were otherwise mostly the same.

The drives had a simple, mechanical write-protect sensor. If the edges of the plastic envelope were intact, putting a disk in the drive would block the sensor, and the drive wouldn't allow writes. But, if there were a small notch cut in the edge, aligned with the sensor, the disk would not trigger the write-protect mode, and you could write to the disk.

The single-sided disks had a notch cut in one edge of the envelope to allow writing to one side of the disk. But, if you cut a notch in the same spot on the opposite side of the envelope, you could disable write-protect mode on the flip side of the disk. A hole punch was the easiest way to make the notch. Voilà! You could store twice as much data on the same disk.