I beg to differ, Derleth. As DreadCthulhu said, it was actually a method of underclocking the CPU to provide backwards compatibilty with hard-coded games.
[On preview, I see that scr4 has beaten me to the “its not overclocking” punch. Still, this post is going out because it has numbers, dagnabbit].
Gather round, young’uns, for some early PC history.
The original IBM PC and XT used 8088 CPUs running at 4.77MHz. There soon appeared clones running at 8MHz. This was not, however, an example of what is now known as “overclocking”. The CPUs used were specified to run safely at the higher clock frequency. After the IBM AT was introduced in 1984 with a 6MHz 80286, clones appeared with 8MHz/10MHz/12MHz etc CPUs.
The “Turbo” button found on clone PCs was, IIRC, a normally-closed toggle switch. If the switch was not connected to the motherboard, the CPU ran at its maximum speed – as also occurred if the switch was connected and pushed in. However, if the switch was connected and released, the CPU ran at a clockspeed of 4.77MHz, to provide backward compatibility with games that were hardcoded for the original IBM PC/XT.
An example of such a program was the original Microsoft Flight Simulator. This – again IIRC - was not even a DOS program. It booted up from floppy, but was its own operating environment. The floppy disk did not have a DOS-readable file system. Its peculiarities were such that it was used as the “acid test” for PC-compatibility. If a PC could run Flight Simulator, it was a true “clone”.