In short the theory is that if you redirect an internal page to your homepage the best way to make sure those links still pass anchor text is to use similar anchor text in your internal links back to your homepage.
I started noticing that I was only ranking on the homepage for certain terms as a result of redirecting old pages. What was it that was causing my homepage to rank for some terms but not others? After much digging around, I came up with the following theory – Google doesn’t just look at page content to determine whether a link is trying to cause a Googlebomb. Google also looks at other onsite factors, such as internal anchor text. If for example URL A has lots of external links that say “miserable failure” but URL A has no content to match that anchor text, then it’s clearly being Googlebombed. However, even if URL A has no content to match the anchor text, it can satisfy the same criteria through internal anchors – perhaps an internal anchor that just says “miserable” and points at URL A is enough to tell Google that the external anchors shouldn’t be considered a Googlebomb.
How did I work out this was happening? I had some legacy pages on my website (I still do), that have odd bits of anchor text pointing to my homepage. Wherever these odd bits of internal anchors match external anchors, then my homepage continues to rank. Where they don’t match, my homepage no longer ranks.
Testing this isn’t easy because if you use the same anchor text in your internal links then the homepage will rank for those words anyway, even without the external link.