The difference between the two dimming curves available in the MAC TW1 is not described correctly in the TW1 user manual. Here are the correct details:

Either a "Square law" or a "Linear" dimming curve can be selected in the MAC TW1's control menus. The "Linear" setting means that there is a linear increase in RMS lamp voltage -- not light output -- as the dimmer is opened. In fact, the "Square law" dimming curve is closer to a linear increase in light output than the "Linear" curve.

• The "Linear" dimming curve is relatively flat at low light intensity
  levels and steep at high intensity. This gives finer control at low
  intensity but coarser control at high intensity. 
  
•  The "Square law" dimming curve is straighter than the "Linear" curve.
  This means that control at low intensity is not as fine, but control at
  high intensity is finer compared with the "Linear" curve.