samwisegamgees: If I remember right the turbo button was for machines with DX processors and the turbo button turned the maths co-processor on and off.
brouer: I don't want to discount the possibility of some vendor having implemented something like that, but I never came across such a button, and I've worked on a lot of different PC brands.
The turbo buttons were always linked to clock frequency.
My PC 20-III didn't have a button, but you could set the frequency by software.
I also remember
atslow.exe and similar.
You are correct! It has been a while and my memory didn't quite serve me. I remember now that my mighty 486 DX2-66 could switch between 33mhz and 66mhz modes. I am still not sure why, maybe it was because of power consumption or maybe because of overheating? I guess it could also be to ensure older programs ran properly.