Rannoch (another fan of Maxwell) mentioned something similar in his interview the other day
For the Walkout people start on all fours and simply walk their hands out (like using an ab roller) the ideal is to get to full extension, hold and then walk back. A single rep is usually enough to show up any weakness in engaging the core. From these simple drills we can start to engage the body as a single unit.
You can do so much with bodyweight.