Essential Core Exercise - Standing Rollout
The point of this exercise is to use your core to resist a dynamic force. As you lean forward and move your arms above your head the force of gravity is going to be increasingly felt on your midsection. Without strong abs and deep core muscles your hips will sag towards the floor.
Your goal is to resist this 'sagging' so only go as far as you can while maintaining a rigid core and as soon as you feel you've lost control of your midsection (you lose tension and it sinks even slightly towards the floor) you've gone too far. Squeeze your abs like you're bracing for a punch but try to create the contraction around your waist, rather than just in the front. Remember: your core is more than just your six pack and the more important muscles are the deep transverse abdominis which provide much of your core stability.