Task mode defines how parents behave. The task mode can be selected from task context menu.

In case the parent is resized or moved in such a way, that some child goes out of parent boundaries - a conflict will be highlighted for your information. In example below, a child starts one day before the parent starts. You can fix the conflict either by moving the child or by resizing the parent.

In case you decide to use modes other than bottom-up, add column Mode to the chart. It will show selected mode for each task. For example, below all tasks are in bottom-up mode (arrow), except for one task. The manual mode task shows a hand icon.