From a plank position, lift one arm and pull a dumbbell or similar under your body from the opposite side. Replace that arm on the floor and use the other arm to pull the weight back to the original side. You can use a band instead and complete all reps on one side before switching.
 
Purpose
The plank pull-through adds difficulty by creating instability and forcing the trunk and hips to resist rotation and changing forces.   
 
Programming
The plank pull-through can be done at the end of training sessions for 3-5 sets of 10-30 reps.
 
Variations
The plank pull-through can be done using a band rather than a dumbbell or other weight.