I've done a lot of work with gestural/interactive motion like this. One area that people tend to make naive mistakes in motion design is directly driving the p

I've done a lot of work with gestural/interactive motion like this. One area that people tend to make naive mistakes in motion design is directly driving the playhead with something like drag distance. For instance, if you drag down on the home screen of Android, stop, and hold; you can see the transparency of the homescreen icons and of the Quick Settings tiles are both controlled by how far you've dragged. The opacity of those icons is determined by your arbitrary drag distance, instead of something more meaningful (like which direction the transition will animate when you let go). One of the nice aspects of this menu is that it's always clear which item is active. You can't drag to an indeterminant state between two items.