When you reach a totem with gems inside, you need to spin the spinner and land the pointer in the green zone to steal a gem. The pointer lands randomly, so it all depends on your luck. The default parameter is 50/50 for the red and green zones. If you die a lot while trying to reach a totem, the red zone will be much larger. To increase the green zone, craft a throne with a better Gem Steal Chance or make a flawless attack.