That is just from being lean (I’ve never been over 7% bodyfat) and is probably from years of sprinting. As far as specific abdominal work… I do abs 2-3x per week at the end of my workouts. The exercises I use are all nautilus style machines (weighted) or Decline Weighted Situps for no more than 20 reps. I also do Landmine Rotations with the Barbell. To be honest with you the reason I do abdominal work is for aesthetic reasons (I want my 6 pack to have more pop) although it is probably helping my bench and deadlift.