It's not true. You can't just guarantee someone X amount of inches like that. Everyone is different. Is he saying that he can take someone that has just about maxed out their vertical and add 6 more inches? How about someone 90 years old that can barely walk?

Most people that have had little or no vertical jump training (or some other training that was similar) can gain 6 or more inches with a decent vertical program. Most people have the same results with Jumpsoles. But you can't guarantee that and there is a big difference between a person with no prior training and someone with a lot.

Also I believe that it's very hard to get your money back if the guarantee doesn't come true for you. So he really has nothing to lose.