When entering activities the flexible way, meaning without defining any fixed start or end dates, you get them all aligned and starting on the current date.

Supposed you link them by a finish to start linkage, you will see the successor activity jumping to a starting point after the end of the predecessor.

Now let’s say you have two other activities, needing 1 week and 1 day work accordingly.

Link them by a finish to finish linkage and see what happens. The successor will jump automatically near the “end” of the predecessor keeping its work expectations as duration.

How does Merlin know, that it had to optimize the successor activity? This is controlled by the enabled “optimize” duration option.

If you prefer to leave the activity to the current planing date, simply disable this option. The activity D will stay on its planned date. Its duration may vary but not its work.

You may want to check attached screencast