Is there anyone who has a very good working and practial knowledge on the stability ball??

Absolutely. I use it all the time and advise my clients to use it as well. Just by sitting on the ball alone one engages the core, and add to that different variables such as bringing the feet closer together; challenges balance and Proprioception.This is just a basic mount. I actually perform some of my dance choreography while sitting on the stability ball and that allows me to utilize my core while taking the some of the pressure off of the lower back and forcing me into corrective postures and steps.I have to say it is a must have in every fitness enthusiasts arsenal.