The video is self-explanatory, but here is some additional info:

• The poses are, as can be inferred, in the pose library.
• I think the problem is this: When I saved hand-on-hip pose, I only saved upper and lower arm bones, which only saved the transformation of those bones and not the hand itself. So, during interpolation, hand naturally interpolates to default standing pose.

Saw in a Youtube short about Blender