The setup pose has the hoverboard off the to left. Press ctrl+H
to unhide the bones so you can see them in setup mode. When you change animations t the hoverboard animation with a mix duration in the Preview
view, the hoverboard interpolates from the current animation to the hoverboard animation pose. If the current animation doesn't key the hoverboard bones, then it'll be in the setup pose for that animation.
Note we have a page that describes the spineboy project in detail:
Spineboy example
And pages for all the other example projects:
Spine: Example Projects