To search using an abstract, simply select "Abstract Search" in the bottom right over the search area (see screenshot below). Then paste in the abstract into the box, and then press 'Enter' on your keyboard.
The abstract search is a very simple, but extremely effective, way to search for publications with a specific scope of interest. Instead of trying to search for a series of keywords in combination to describe the area you are interested in, you can simply find an abstract which represents the area well, and copy and paste this into an abstract search. The results identified will be closely determined by the contents of the abstract. Abstracts of methods papers or highly technical wording will be likely to bring back a smaller number of results which are highly related in scope to the entered Abstract. Using a review paper abstract, or less technical piece of text is likely to lead to a greater number of more general results.