The side / rotation core pattern the book wants alongside the crunch and leg raise. Trains the obliques and quadratus lumborum — also bulletproofs the low back from the side.
Cues
In a hyperextension machine set sideways (or standing with a DB in one hand), raise the torso to the side against the load through a full range. A cable woodchopper covers the rotation version.
⚠️ The book warns this one makes you very sore — start light.
Programming
2–3 × 12–15 / side. One of the few moves you needn't take fully to failure.