When you reach a totem with gems in it, 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 red and green zones. If you die a lot while trying to reach a totem, red zone will be much bigger. To enlarge the green zone, craft a throne with the better Gem Steal Chance or make a flawless attack.