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