Annotations that are on a tier without a parent tier (top level tier) and that have no or only depending annotations on symbolically associated tiers can be split in two ways:
To split a annotation exactly in the center, select a annotation and click on.
This will split your annotation exactly in the center and both the annotations will have the same values.
To split a annotation at a specific point, select a annotation and right click on the point where you want to split the annotation and select.
This will split the annotation at the point where the right click is made.
You can also split an annotation (in one of the ways described above) which has only time aligned depending annotations (symbolically associated). In this case the annotation will be split together with its depending annotations.