The animation works because Eugene is on bones that are Path-constrained to the middle of the long spline.
Only the start- and endpoints of the spline are animated; the constraints keep the bones at wherever the middle happens to be.
A description of the PRJ (attached above) I wrote up for a bug report...
Here it is, the complete CG animation workflow in just four small bites... and one Puppy Dog Tail!
Part 1 Modeling
Part 2 Bones
Part 3 Animation
Part 4 Rendering