Performing a roll forward during a side plank increases difficulty and activates spinal rotation musculature.

“well then you have the option to make the whole thing more difficult by doing a sort of roll forward where you actually take the part of your shoulders which is on the side of your free arm so the hand that is not pressing into the floor and you rotate forward so that your chest is now aiming toward the floor maybe not to a full push-up position but maybe close and then back again to the side plank what you're doing there is you're activating some of the musculature that is for rotation of the spine”