From supine (or seated), simultaneously lift the legs to a diagonal and roll the torso up to reach the arms toward the toes, balancing on the sacrum/tailbone in a "V" shape. Hold with the powerhouse fully engaged, then roll down with control. Progressions move from one leg, to both legs, to rolling fully up and down into the V.
A signature advanced Pilates exercise requiring substantial abdominal strength, hip-flexor control, and balance (pilates-powerhouse, dynamic-balance-drills). Build up via the pilates-roll-up and bent-knee/single-leg versions first. Avoid gripping the neck or dumping into the lower back; the lift is driven by the deep abdominals.