WTF. Universal Creations has put it exactly right. What does that have to do with the two 50mm f/1.4 Sigma lenses which are simply very different (and none has any relation to IS). But, say, the Sigma 17-70/2.8-4 OS C lens has the same construction in whichever mount, despite the "OS is not available for the Sony and Pentax mount" side note on the Sigma site, and the block diagram is the same. The OS seems permanently disabled in the A- and K-mount units however, and there's no switch.
If you compare the block diagrams of the Canon 70-200/2.8 L lenses - IS and non-IS - you can see that in the IS one there's a 3-element group added towards the rear of the (otherwise broadly similar) optical assembly.
Non-IS:
IS non-II. The group isn't marked as IS in the site's block diagram...
... but the block diagram of the IS II version, while having more special elements, reveals a similarly shaped group now marked as being IS:
Peace.
If you compare the block diagrams of the Canon 70-200/2.8 L lenses - IS and non-IS - you can see that in the IS one there's a 3-element group added towards the rear of the (otherwise broadly similar) optical assembly.
Non-IS:
IS non-II. The group isn't marked as IS in the site's block diagram...
... but the block diagram of the IS II version, while having more special elements, reveals a similarly shaped group now marked as being IS:
Peace.