Hook your feet under the foot pads of a declined bench and lie supine (on your back).
Place your hands behind neck.
Exhale as you raise your torso by flexing your waist and hips. As your torso rises, twist your waist to one side.
Inhale as you reverse the motion and lower your torso to the starting position.
Repeat, this time twisting your waist to the opposite side.
Keep repeating and alternating the side to which you twist your waist.
Comments and tips
Keep your neck in a neutral position.
Make the decline twisting sit-up easier by either crossing your arms on your chest or performing the exercise on a flat bench.
Make it more difficult by holding a weight plate either behind your head, in your hands, or on your chest.
If you perform the twisting sit-up on the floor with your hips already bent in the starting position, your pectineus, adductor longus, and adductor brevis will not act as synergists in the hip flexion.