I wouldn’t recommend that article as it offers some dodgy information (position:relative on table-cells doesn’t work in Firefox 35) and the point about adding position:relative to negative top margin misses the point of why the position relative is needed (it’s about stacking contexts and not a fix to make the margin-work ).
The article also makes no mention about collapsing margins, margins on elements following floated content, margins on cleared elements or margins on the opposing edge of a floated elements (which create negative space).
All of which are needed to understand properly the impact of negative margins.