You have to set 2 people as tandems, say tandem 3a and 3b. 3a will start when the third spot in your rotation comes up (assuming he is rested). 3b will be the first reliever that comes in that game (assuming he is rested and availability is set where he is available that inning). Tandems must be at the end of your rotation, meaning you can’t have tandems 3a/3b then starter 4.
This setting is particularly useful when you have 2 guys who have 3-5 ip/g. They can work together and fill the innings of one normal starter.