Sit on a flat or declined bench with your trunk only slightly flexed and your feet anchored securely. Flex your hips and knees. Keep both legs aligned straight and close together. Begin with the upper body lowered just enough to provide tension on your abdominals, arms crossed over your chest, and hands braced under your chin.
Slowly lower your upper body while keeping your trunk slightly flexed. Continue to lower your upper body to the point where your trunk muscles are challenged to hold positioning. Hold, continue to contract your abdominals and hip flexors, then slowly raise your trunk back to its original start position. Repeat.