Electrical Wizards ?

I have a really, really dumb question. A friend of mine asked why he
could not "Dial down" a variac and use it to supply power for a stepper
driver system. I could only conclude that there is the chance it could
not maintain the voltage accurately, in fact even fail miserably,
because it is nothing like having Primarys and Secondarys of a
transformer. true ??

Then I was wondering what actually happens if Variacs are used to supply
a transformers primary winding. If it was all setup, and running some
stepper drivers and you dialed down the voltage, What do the Stepper
motors do ?

Slow Down ? get weak ? What ?

I slept thru most of electronics class.... now I pay for it.

thanks,

Chris L