To being, position yourself in a front plank with your hands resting on a mat and both arms fully extended. Extend your legs behind your (knees locked) with your toes resting on two glider discs (paper plates work well too). Check to make sure that your mid section isn't hiked up (your butt should be in line with your shoulders and heels). Keeping your body in a straight diagonal line, bend your right elbow until your forearm is resting on the mat. Then do the same with the left arm. As you bend your arms your body will slide back on the glider discs. Then, engage your core and extend your right arm then left arm (your legs will slide back in) until your back a the start position with your arms fully extended. Repeat this motion for 15 seconds, then switch to lowering the left arm first followed by the right for another 15 seconds.
For a better visual of the exercise, check out the video clip below.
RunnerDude's Weekly Exercise: Dolphin to Elbow Plank from Thad McLaurin on Vimeo.