Pressing "B" in Pokemon is used when you don't decide for your Pokemon to evolve - or just by accident. But how do you resolve your mistake. Easy!
If you press "B" within the evolution process, your Pokemon will stop evolving. But, do not worry, the next time it levels up, the evolution will resume; unless it reaches lvl. 100, therefore making it impossible to evolve then. If pressing "B" was the key to stop evolving the Pokemon forever, there would be no use of the "Everstone".
Hope I helped! :)