This action event can be used to wait until a specific asynchronous object transition effect has finished playing. You have to select the object for this action event. Hollywood Designer will then block the execution of the project until all transition effects on this object have finished playing. Obviously, this action event only makes sense when using asynchronous transition effects because synchronous effects will block anyway.
To wait for all asynchronous effects on the current page to finish, use the Wait all FX action event instead.