This pullover variation produces constant tension, one of the major keys to muscle growth. It's a great core and upper body movement for thoroughly taxing the targeted musculature.
The secret is to keep your core fully engaged throughout by trying to minimize the amount of arch produced in your lumbar spine and keeping the low back pushed into the floor, particularly in the contracted (bottom) position.
It's also an excellent option for lifters who have excessive lordotic curvature throughout their spine.