All the PCs I have personally used or encountered which had a "Turbo Mode" (clock frequency switch, with frequencies in the 5-25MHz range) did this via a hardware button similar to the reset button.

However, this question links to a forum thread where somebody claims that this was possible via the keyboard (and on a PC that apparently has no hardware turbo button, judging from the images I googled).

Were there any PCs where "Turbo Switching" was possible via the keyboard, and which ones?

If yes, were the hotkeys intercepted by the BIOS, or was this some DOS extension, driver, or TSR?

If it was intercepted by the BIOS, how did they deal with other programs that wanted to use this particular key combination?