The strength ball is a popular favourite, & this model has LEDs inside which only work when it rotates at speed. For the benefit of those who have not seen it before, the strength ball uses gyroscopic force to encourage muscle building exercise. The rip cord is inserted & pulled to start the central weight spinning, after that it is up to the individual to rotate it faster & faster. RPM is recorded on the digital counter enabling a competitive element between friends. There is a knack to it but definite benefit can be felt after just a few goes & it is extremely addictive.