Turbo button
On IBM PC compatible computers, the turbo button selects one of two run states: the default normal speed or a reduced "turbo" speed. It was relatively common on computers using the Intel 80286, Intel 80386 and Intel 80486 processors, from the mid 1980s to mid 1990s. The name is inspired by turbocharger, a device which increases an engine's power and efficiency. Contrary to what it suggests, the "turbo" button was generally intended to let a computer run slower than the speed for which it had been designed when pressed.

Purpose 

With the introduction of CPUs which ran faster than the original 4.77 MHz Intel 8088 used in the IBM Personal Computer, programs which relied on the CPU's frequency for timing were executing faster than intended. Games in particular were often rendered unplayable. To provide some compatibility, the "turbo" button was added. Disengaging turbo mode slows the system down to a state compatible with original 8086/8088 chips.

Switching 

On most systems, turbo mode was with the button pushed in, but since the button could often be wired either way, on some systems it was the opposite. On some systems the turbo button was linked to a turbo LED or two-digit segmented display on the system case, although in almost all cases, the indicated frequency (in MHz) wasn't a measure of the actual processor clocks, but the two "fast" and "slow" display options set by jumpers on the motherboard (or a daughterboard linked to the button), or on some later machines, set in the BIOS software.

Some systems also supported keyboard combinations -- and -- for switching turbo mode on and off; ITT Xtra used -- to toggle.

Some keyboards had a turbo button as well, located near right Shift. Unlike the turbo button that was common on computer cases, the turbo button on the keyboards did not control the clock rate of the CPU; rather, it controlled the keyboard repeat rate.

Use 

The feature was relatively common on systems running 286 to 486 CPUs,
and rarely on first generation Pentium CPU equipped computers. The frequency displays largely disappeared or were reprogrammed to display "HI"/"LO" when CPU speeds reached 100 MHz, since most systems only had a two-digit display.

As new computers continued to get faster and had a wide variety of available speeds, it became impossible for software to rely on specific CPU speed. As software began to rely on other timing methods, turbo became mostly irrelevant to new programs.

Software implementations 

While the implementation of an actual hardware turbo button has disappeared on modern machines, software developers have compensated with software replacements. One example is DOSBox, which offers an adjustable emulation rate. Modern PCs that support ACPI power management may provide software controls to switch ACPI performance states or other CPU throttling modes. This is used for power saving or to prevent CPU overheating rather than for compatibility because modern applications use the real time clock for timing instead of the CPU clock.

References 
Turbo PC by Complete Business Systems, @1991 - 1994, Ira Kleiner

Computing input devices
Computer keys