If you have ever seen one of those crazy balls of scrap iron being swung around by a Russian giant then you have seen kettlebells being used before. The facts are that kettlebells have come back in style after taking a few decades off, and there is no more effective way to build up your core explosiveness as an athlete.
The original use of these bowling-ball-with-handle contraptions dates all the way back to 18th century Russia. It was here that weightlifters, professional strongmen, and the Russian military devloped the workout strategies that made them famous. They also unwittingly developed a simply blue print for athletes of the future that wanted to get back to basics and see real results from their workouts.
The man reason why kettlebells are so effective is that they provide a resistance that it off-balance, and not perfectly balanced like a dumbbell. This makes it far more effective for building functional strength.