First let me say that I was a fan of the bullworker as a kid and this is the same thing. It is isomertric exercise which definitely does build muscle. I would rate this product 5 stars but becuase of the horrible chart it came with can only give it three.
The reason why the chart is so bad is because in several instances the pictures don't match the exercise. I could not believe they would not fix this before selling it to the public. The point of having a chart with pictures is that once you get used to the workout, you can just look at the pictures and proceed, however, sadly after I noticed several of the pictures were duplicates, I read the paragraph below and realized they were describing a completely different exercise. Very poor practice for a product that is very good.