A relief win is defined as any win by a pitcher who was not the starting pitcher. Relievers can earn relief wins in two different ways -- one far more common than the other. First, if a reliever is in the game at the time his team takes the lead for good, he is credited with the victory. A reliever can also pick up the win if the starting pitcher pitches fewer than five innings in what would have been the starter's win, and the official scorer deems that reliever to have been the "most effective" in preserving the win.
Relief victories are inevitably a product of circumstance; they come down to when a pitcher enters the game. Often times, a reliever will allow the tying or go-ahead run to score before his team takes the lead for good in the next half inning. In this case, the reliever didn't do his job very well, but he picks up the win anyway.
In a rarely used clause, an official scorer can deem a relief pitcher's appearance "brief and ineffective." (For example, if a reliever relinquished a one-run lead by allowing three runs, but was still in line for a win after his team scored four runs in the following inning -- that may qualify.) In such cases, the scorer can award the win to a pitcher who followed that "brief and ineffective" pitcher. The pitcher who receives that win is also determined by the official scorer.
In A Call
"wins in relief," "wins out of the bullpen"