This exercise works your posterior chain like no other. It absolutely hammers the lower back and traps in this variation, as the range of motion is very short. Get into a tight position (arched back, shoulder blades pulled together, knees slightly bent, abdominal pushed out against your belt). This is the starting position. The key is to emphasize the arch in your back with this movement. To get maximal benefits from this, you need to be arching as hard as you can the entire time. You'll definitely know by feel if you're doing it right. Then, you'll flex at the waist, while holding the arch in your back. Descend until you feel as if you're going to lose the arch, then return to the starting position. Repeat until you've reached the desired number of reps, or your back dies, whichever comes first.