Article rewriting techniques follow a tree structure like this: You may rewrite on an article, paragraph or sentence level.
Rewriting an article as a whole has the advantage that you get a better grasp of the topic and concepts presented. This way you can deliver better rewriting results.
A common article rewriting technique is to paraphrase on a paragraph level. This is a solid practice, since each paragraph has supposedly a separate message that it intends to convey.
Another article rewriting method, most common with article spinning software or writers that are in a hurry, is to rewrite on a sentence level. This technique mainly consists of replacing words and short expressions with synonyms.