A hammock task is dependent on external dates for its start and finish dates and, ultimately, its duration.

The name, hammock task, is derived from the way a hammock's shape is determined by the distance between the points to which it is attached.

1. Create the task to become the hammock. 
2. Go to the task that will drive the start date of the hammock and create a Start-to-Start relationship with the hammock.
3. Go to the task whose finish will drive the completion of the hammock and create a Finish-to-Finish relationship with the hammock.

By doing this, the hammock (sumary) activity will be representative of the underlying / linked set of activities.