From BR Bullpen
A swingman is a pitcher who alternates between the bullpen and starting rotation, being used regularly in both roles. Such pitchers have usually been used as combined spot starter/long reliever, such as the 5th man in a 4-man or 4-and-a-half-man rotation. The role's usage decreased over time with larger pitching staffs and more rigidly defined pitcher usage. The only common usage of swingmen in today's game is in the early part of the season, when some managers will take advantage of the increased number of rest days by using their 5th starters as swingmen.