General SAT is hard. If clauses can have any number of variables, no fast algorithm exists.
The problem is NP-complete. -SAT restricts each clause to exactly two variables. Every clause looks like "a OR b", "NOT x OR y", or "NOT p OR NOT q". With exactly two per clause, everything changes. You can solve -SAT in linear time using graph algorithms. That is what this section is about. The restriction to exactly two variables makes the problem tractable.