Core strength is fundamental to pilates philosophy and the oblique sit up is a very important way of strengthening the underlying abdominal muscles that can be forgotten when doing standard crunches or exercises simply focusing on a “six pack”.
While the focus of the oblique site up is not on the common six pack muscle groups it is essential for good abdominals to exercise the oblique muscles as they are what gives the flat stomach.

The main purpose off this exercise is however, core strength.

The exercise:

Lay flat on your back with hips parallel to the floor.
make sure that your shoulder blades are flat on the floor.
Roll your neck from side to side.
Tuck your chin in.
Take a deep breath.

oblique sit up

Place the fingers from both hands behind the ears.
While exhaling gently raise one elbow towards the oposite knee being careful not to lift with the arms.
While moving it should close the gap between the pelvis and lower rib cage.
While inhaling slowly move back to the start position.
Repeat for the other side.
Repeat 5 times for each side.

Important:
Do not apply any pressure with the arms.
If the neck hurts stop immediately.