...this might be inaccurate (my 'research' was minimal, to say the least) but it appears that gas collects in these areas, displacing them and causing pain (the displacement we can see in the grid).
This might be fanciful, because the very nature of the parts might be enough to justify their inclusion (they can all do the...!) but as this is one of Ifor's, it's the kind of detail we might expect. And I like this one, but it could go the same way as those 'thematic names', of course.