Main focus: Arms, shoulders, chest and upper back.
Without CoolBoard: Traditional Dips give you hands a solid base so the only work being done is in lifting your body weight.
With CoolBoard: Now your hands can move in all directions. Your body must control and minimise that movement so more muscles must do more work. Your core and many stabilising structures must constantly work to hold the CoolBoard steady while executing the Dip. Think gym ball training on steroids!