The search engines find your hub through external links coming to your hub. So if you post a comment on a blog entry that is related to your hub and you leave the link to your hub (or your hubpages profile) in the link field, then Google will follow that link when its spiders crawl over the blog post on which you've just commented, covering your hub and crawling out across the links that you have posted on your page.
A lot of what we used to think we knew about SEO is red herrings. The tags were always meaningless and only helped to connect related pages to one another. Your best bet with attracting search engine traffic is to write a high quality and engaging hub that your readers enjoy enough to pass on to other people via Twitter, Facebook and other social networks, and which people feel are good enough to link to from their web pages (or other hubs), thus giving you a boost with search engine traffic.
It has to be above board; Google does punish black hat techniques as much as possible, and the most recent updates have really penalized some of the bigger sites (HP included).