The word "core muscles" have been over-used and taken out of context.  Core muscles (specific to riding and racing) include the muscles from your chest to your pubic bone - the sides of your abdomen and your lower back muscles into your butt muscles (glutes).  This exercise will isolate the muscles on the side of your abdomen.  

Strive to complete 10 reps per side - with controlled movement | 2 Sets - no rest in between sides (10 reps right side - 10 reps left side - Repeat)

If you have any questions or need anything clarified, send me an email at Contact@CoachRobb.com

Yours in sport and health,

Coach Robb - CompleteRacingSolutions.com