Benefits:
Great beginner exercise for core strength and spinal stability.
Movement:
Lift and extend one leg to the floor behind you. Place the ball of your foot on the floor as in a push-up position. Maintain a neutral spinal alignment in this semi-supported position. If you feel comfortable enough, extend both legs into a full plank position. Be sure to keep your body straight and not to drop your hips during the exercise. Your elbows directly beneath your shoulder blades.
For increased difficulty and tension you can shift elbows forward.