Hammock Task???

How to Build a Hammock Task
A "hammock" task is dependent on external dates for both 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.

As scheduled dates in other tasks change, the duration and start and/or finish dates of the hammock task change as well. In other words, given three tasks A, B, and C, task B must start on some date from task A (date A) and task B must finish on some date from task C (date C). For an illustration of this task B, see Example 1.
Example 1--Hammock Task B
A
--------------C
------B-------
The span of time between dates A and C dictate the available time to accomplish task B. As Date A changes, the start date for task B changes as well; If date C moves, task B must finish appropriately. The difference between dates A and C is the duration for task B. See example 2.
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Example 2--Date A Late, Date C Early
A
-----C
--B--

To build a hammock task between two other tasks, follow these steps:

1. Create or determine three tasks that correspond to A, B, and C in the example above.
2. Highlight the cell that contains date A (the date that will determine when task B starts).
3. On the Edit menu, click Copy (Cell).
4. Highlight the cell that contains the Start date for task B (the hammock task).
5. On the Edit menu, click Paste Special, and click Paste Link.
6. Highlight the cell that contains Date C (the date that will determine when task B finishes).
7. On the Edit menu, click Copy (Cell).
8. Click the cell that contains the Finish date for task B (the hammock task).
9. click Edit, click Paste Special, and click Paste Link.

Notes and Cautions

• Do not specify a predecessor for a hammock task. Only Paste Linked dates will determine start and finish. You may specify predecessors for the tasks from which dates are paste linked into the hammock task.
• A negative duration (finish date is before start) is not allowed. Depending on the source of the dates, the hammock task may turn into a milestone (zero duration) or erroneously calculate duration before the finish date or after the start date.
• An OLE update is different from a calculation. You may not see expected changes in the hammock task after calculating project. From the Edit menu, click Links to edit/update OLE links (Paste Links). Alternatively, press the F9 function key twice.
• Because the hammock task relies on other tasks for start/finish information, anything that affects those other tasks (resource leveling, constraints) will ultimately affect the hammock tasks dates.
• If the hammock task is a child to a summary task, it may inherit predecessor behavior from the summary task that may affect its duration and/or start and finish dates or create a circular relationship.

Reproduced from Microsoft Support site