Many a time, you would have come across sites (especially the large ones) where the deeper you dig into the site hierarchy, you can see the Pagerank toolbar grayed out or having a value 0. In general, the home page is the starting point for a website and it accrues the maximum Page rank.
The entire domain’s authority and trust is reflected by this page rank value. The home page then tends to distribute this page rank to the first level (categories), the second level (sub-categories) and the third level product pages which we often refer to as link juice. In general, the first level pages tend to derive the maximum link juice from the home page. But in a site with excessive number of sub-categories and product pages (money pages), the pagerank distribution is not proportional with some gaining link juice and a large majority not gaining any.
I am not delving into pagerank sculpting which was the rage of the SEO industry not long ago. This method was adopted to make certain areas of a site more powerful by flowing link juice from the home page preferentially to pages of interest (possible commercial intent). The internal linking architecture is altered from its natural form to highlight more important pages than the rest on the site.
In my opinion, internal linking with appropriate anchor text can be used to improve the ranking of pages within a site but this has its limitations. Integrating a blog with the main site so that it forms part of the global site template is vital. The different posts on the site blog can point to different pages of the site to flow some much needed link juice to prop select pages up and assist them in getting indexed by Google and ranked on the SERPs.
Eric Enge at Stone Temple Consulting has done some research in regards to distribution of domain authority and trust to pages on the lower level pages of a site. Let us assume you have a site selling products. You have the home page, many categories with respective sub-categories and product pages in that order. Let us consider an example where you sell Canon digital cameras as shown below.
The specific product page in question is listed on Page 4. You are not happy as this product page has not recorded any page rank. If it does not have the requisite page rank, there is a possibility that it will not gain entry into the Google index.
In this scenario, it is evident that the site despite a great PR of 6 is not able to flow link juice into this specific product page. Eric has found out in his experiments that an inbbound link to this deep page with appropriate anchor text (in this case EOS D7 Canon Digital Camera Review) from an external site is a huge help to get this product page into the index and help get it ranked. The link may not be of superior quality yet it helps a great deal.
As we all know, every inbound link is a vote to the site and the search engines recognise this as a strong signal. The search engines adjust their algorithm accordingly to flow more internal link juice into the page which has gained inbouind links. The more the merrier seems to be the case.
In our example, the Canon EOS D7 product page ends up gaining link juice thus helping it enter the Google index and get ranked over time. Deep inbound links help a site distribute more authority and trust to pages that receive such inbound links. This is a classic case of an inbound link triggering a tunnel of sorts to flow a site’s internal link juice to select pages at a lower hierarchy level.
In summary, you do not need masses of links to get your third level or product pages indexed and ranked on the SERPs. Just a few inbound links will facilitate better flow of the site’s domain authority and trust (read link juice) to the specific page recipient of the inbound links in question. You can read the post on impact of deep inbound links on a site’s lower level pages  on Eric’s site.
Ravi Venkatesan is a senior SEO consultant  at Netconcepts, an Auckland search engine marketing company  offering both seo and pay per click services  to their customers in New Zealand and Australia.