LInkjuice isn't a technique. It's just what's passed on from a backlink to the source URL. The factors passed could be Authority, Pagerank, Relevance, etc.... all of these in the form of something known as 'Linkjuice'. It is a hypothetical unit but is used throughout as a term in SEO. E.g. When two external Pages(A & B) link to you. -> A has a pagerank of 6, is keyword/niche specific to your page to which it is linking and is on a domain which is 10 years old. -> B has a pagerank of 2, is not in your niche and is on a domain that is 2 years old. In such a scenario, the page A passes more Linkjuice to your page than the page B.
Linkjuice is nothing but the outcome of the backlinks that you have created. It means if you have done backlinking on some websites then how much is it getting you the reward, just like getting the juice out of a fruit!