This is where the most amount of familiarity lies. A recruiting firm provides candidates and is paid for their efforts once the Client hires a presented candidate. In reality the recruiting firm is working for free until a hire is made. What the Client doesn't know is that the recruiting firms goal is to secure thousands of jobs from as many clients as possible, thus once a candidate is found, there is a high probability of being able to match the candidate to an open job. The recruiting company doesn't prioritize or actively work any specific job, its just 1 of 1000, and they hope to have a candidate come along that matches. If they are unable to fill the position, its no big deal since they were not paid to do so - ultimately there is no sense of responsibility.
Typically thought to be more expensive than contingent searches and reserved for executive level searches (both of which are wrong), is a highly prioritized, targeted, and aggressive search by the recruitment firm. The firm commits to filling it within a specific time period, partners with the Client to ensure success and takes responsibility for filling the position. For every retained search, it receives:
When you have an urgent opening that needs to be filled quickly, a retained search is the most reliable way to ensure it receives the attention it needs.