When done correctly, sit-ups are an effective and safe abdominal exercise. The characteristics of an effective ab exercise are that the movement bends the spine in it's full range of motion while contracting the abdominal muscles from full extension to full flexion.
Our standard sit-up uses an AbMat. Placing the AbMat under the lumbar curve allows for a contraction of the abdominal muscles from full extension (when spine is arched or extended 15-30 degrees) to full flexion (when the lower back comes off the floor and spine is about 30 degrees from the floor). Furthermore, turning the knees out with the legs bent puts the hip flexors at a disadvantage, so you will really be using your abs, and not your hip flexors to complete the movement. For a more in depth explanation of how and why the AbMat works, click here.
Requirements for the sit-up: shoulder blades touch ground at bottom of rep; come to a full sitting position at the top.
Here is a video highlighting the basics of the sit-up: