09:43 Jul 4, 2011 

... on just what form this 'control' takes — is it a rotary knob, a push-button, a slide switch, etc.?

I think you may need to find that out in order to get just the right term.

Also, is this indeed an actual control? Are you implying that there are controls for both 'fixed' and 'variable' power? Seems a bit odd... isn't it simply fixed power by default unless you turn on the 'variable power' control. Unless of course it isn't an actual 'control' at all, but simply means that it features control of the power that is fixed, which is another possible interpretation of this expression.

I think more context may be the only way to tell.