As everyone has already mentioned, there's no "standard" way to implement the turbo/non-turbo mode functionality.
For example, the VLSI VT82C486 chipset (for 486DX and 486DX2 class CPUs) has this to say about its implementation:
TURBO/NON-TURBO MODE CONTROL
It has become standard for fast PC/AT-compatibles to provide means to slow operation for older speed sensitive software. This is especially true for graphics intensive entertainment software which may otherwise operate much too fast on a high speed machine. One way this mode may be toggled on and off is by external control of the TURBO input pin. The Slow Mode is activated and the VL82C486 generates continuous invalidates (via -EADS) to the CPU when TURBO is low. When TURBO is high, the -EADS signal is not modulated. This feature provides the capability of emulating the effect of running at slower CPU clock frequencies even though the clock frequency is not changed. It is provided in addition to a method of actually changing the CPU clock frequency.
The TURBO pin is normally connected to the keyboard controller and triggered by the BIOS via detection of a key combination such as Ctrl Alt + / Ctrl Alt -. This input is often externally ANDed with a mechanical Turbo switch on the front panel.
So the VLSI implementation activates the -EADS signal of the 486 CPU, which causes it to execute an "invalidate internal cache" cycle instead of whatever normal cycle it would otherwise execute. Basically stealing clock cycles from the 486 by keeping the external bus occupied with bogus invalidate cycles. It offers a choice of how much to slow down the CPU, too: full speed (no holds), 50% (a hold every other clock cycle), 66.6%, ... all the way up to 87.5% (seven holds for every eight clock cycles).