Designate the first vertex for a Bezier path

To animate a path, After Effects designates the topmost vertex at the initial keyframe as the first vertex and numbers each successive vertex in ascending order from the first vertex. After Effects then assigns the same numbers to the corresponding vertices at all successive keyframes. After Effects interpolates the movement of each vertex from its initial position at one keyframe to the position of the correspondingly numbered vertex at the next keyframe. At any time during an animation, you can designate another vertex as the first vertex; this causes After Effects to renumber the vertices of the path. Renumbering vertices causes path animation to change, because After Effects then maps the new vertex numbers to the corresponding old vertex numbers still saved at successive keyframes.

Some shape path operations, such as Trim Paths, also use the first vertex as input to determine how to modify the path.

Note: By default, when you add a vertex to a path, the new vertex appears on the path throughout the duration of the path but reshapes the path only at the time at which it was added. When you delete a vertex from a path at a specific point in time, the vertex is deleted from the path throughout the duration of the path. Prevent After Effects from adding and deleting vertices throughout the duration of the path by choosing Edit > Preferences > General (Windows) or After Effects > Preferences > General (Mac OS), and deselecting Preserve Constant Vertex Count When Editing Masks.

Create an animated path.
In the Timeline panel, move the current-time indicator to the point where you want to designate a new first vertex.
Select the vertex to designate as the first vertex.
Choose Layer > Mask And Shape Path > Set First Vertex.
Note: The vertex designated as the first vertex appears slightly larger than the other vertices in the Composition panel.