That is called hyperextensions.
They are not bad but you can not go above parallel or it will hyperextend the back and not good.
They are more effective if you kindof lay with a pad supporting your abs and the heels are supported and bend from the waist fwd with the head twards the ground then come up slowley but not above a strait line drawn from your hips to your neck.
They hit lower back and some hamstrings too. Generally this is a higher rep exercise as the lower reps can put more of a strain and if you want to go heavy then I would racomend dead lifts.
I do those but in the upper 20 rep range though.
They are good though none the less.
Nice dwg killer
