This overloaded method is available if you want to implement custom turn management logic instead of using default one.
To use it first you have to tell the server that you are not going to use default turn management logic while calling stratGame API. startGame APi also has a overloaded method which will ask you two more parameter, which is as follows:
public void startGame (bool isDefaultLogic, String nextTurn)
You dont want to use default logic then pass first parameter as false and second parameter as the user name whose turn will be first just after starting the game.
Then you can go ahead the sendMove API with the nextTurn parameter to set the next tturn every time a move is sent by the current user.
I hope it helps.