1. Lie on the floor with your legs bent, your feet flat on the floor. If you prefer, you can keep your legs bent at a 90 degree angle so that the bottom of your feet are vertical.
2. Place your hands on the back of your head, or if you prefer, cross them over your chest.
3. Lift your right shoulder blade off the ground moving your right elbow towards your left knee. This results in a "twisting" motion, and is why this exercise is often referred to as a "Twisted Crunch".
4. Feel the squeeze for a second.
5. Slowly return to the starting position.
6. Repeat steps 3 and 4 for your other side (i.e. move your left elbow towards your right knee)
7. Repeat steps 3 - 6 until you've completed the desired number of reps.

3 sets of 25 reps. Try adding weight if you find this too easy and you're trying to build muscle size/strength, or, if you're not trying to build muscle size, just increase the number of reps.

Obliques

Sometimes referred to as: Waist (sides), Upper hip, Oblique muscles

Rectus Abdominis (also known as Abs, Abdominals, front Abs)

Psoas major (also known as Hip Flexors, Hip)