Ab Rocker is the ninth most effective ab exercise according to ACH study. You need ab rocker machine to do this exercise. So you need to buy one.
More info @

http://exercise.about.com/od/abs/ss/abexercises_9.htm

please suggest video for this exercise in the comments

Best Six Pack Abs Training Book with lots of workout routines to get six pack abs efficiently with great meal / diet plans to cut those extra calories and get awesome six pack abs.

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Tagged: abs workout, six pack abs exercises