Found on Intel
-based personal computers, the turbo switch would allow the user to change his CPU
clock speed. After the original 8088
-based IBM PC
with 4.77 MHz
CPU speed was released, a number of clones
turned up, some of them using different CPUs with higher clock speed.
This might sound like a good idea at first, until you realize that everything from bus timing to delay loops depended on a CPU speed of 4.77 MHz.
Pac-Man would run too fast to be of any use, hardware would fail (sometimes permanently] and thus, you needed a way to switch your system from ultra high speed 12 MHz back down to the compatible speed.
Turbo switches used to be a feature found on almost all PCs up to the 486, the earliest models being hardware only, until BIOS manufacturers would create a hotkey sequence for the purpose of speed switching.
An anecdote :
HP once gave stick-on dummy turbo switches away as a promotional gag item - administrators who had received some sometimes even used them to calm down users asking for a faster computer. With success. Stick a switch on, user happy. Just do not take it off again...
A more professional version of a turbo switch was found on the DEC PDP-15. Using a knob, you could run the CPU at full speed or reduce speed down to one instruction every few seconds. (Or - to any speed inbetween.)
Perfect for debugging those Assembler programs using switches and blinkenlights in the process.