Whenever you add a second (or third, ...) milestone to a project, you'll have the option to link the new milestone to already existing milestones.
If you then move the deadline of a milestone that is linked to another one, you'll have the option to link those date changes as well. Therefore, you need to put the slider next to "Propagate date changes" on "Yes".
An example: The deadline of the dependant milestone was originally one week later than the deadline of the first milestone. When you changed the deadline of the first milestone, the dependant milestone moved too and now still is one week after the deadline of the first milestone.
Note: Meetings will not be moved automatically.