The v-sit is an effective abdominal and core exercise that works the rectus abdominis, the external obliques and internal obliques. This exercise also engages the hip flexors.

How to Do It

• Begin in a seated position, contract your abdominal muscles and core, and lift your legs up to a 45-degree angle.
• Reach your arms straight forward or reach up toward your shins as you are able.
• Maintain good core posture and a strong spine.
• Hold this "V" position for several seconds to begin. As you get stronger, hold the position longer.
• Return to your starting position slowly.
• Just before you reach the floor, stop and hold the position for a few seconds.
• Repeat this entire movement several times.

Also See:
Dragon Flag Advanced Core Exercise. Credited to martial arts master, Bruce Lee. the Dragon Flag, is arguably one of the more advanced bodyweight exercises you can do.
The Best and Worse Ab Exercises.
More Ab Exercises
Top 10 Questions About Abdominal Exercises