The turbo button actually was used to underclock the CPU's speed, not increase it.

https://en.wikipedia.org/wiki/Turbo_button

It could work either way. It would be better to phrase it as "outside of turbo mode it operated as a 4mhz 8086, with turbo it operated faster (usually the CPU's actual rated speed)".

Hah that reminds me of the way of a supermarket chain to shill rebates.

Turbo button not pressed, run at lower speed than rated

Turbo button pressed, run at rated speed

No rebate, sell at a huge margin

Rebate, sell at a minimum margin

The linked article suggests it could be both ways for button behavior. But I see what you mean for how it achieves it.

Yup, on many boards you could either jumper the direction (turbo switch closed= slow or closed = fast), or set it in the BIOS.