Difference between Task Callbacks and OnCompleted()

when I was playing with tasks on C#, I was wondering what's the difference between using GetAwaiter().OnCompleted() and callbacks

Case 1 : task1.GetAwaiter().OnCompleted()

Task task1 = new Task(() =>
            {
                //Do Work_1 here
            });

task1.GetAwaiter().OnCompleted(() =>
            {
                //Do something here where Work_1 Completed 
            });

task1.Start();

Case 2 : CallBack

await Task2(() =>
            {
                //CallBack                            
            });
private async Task Task2(Action callBack)
            {
               //do Work_2 here
               await Task.Run(callBack);
            }

I want to understand this and I think I'm missing something.