The pointer of the spinner lands randomly, so it all basically depends on your luck. The default parameter is 50/50 for the red and green zones on the spinner. However, there are certain factors that can either increase or decrease your chances:

1. Health
The more health you have when you reach the totem, the better your chances of stealing a gem.

2. Thrones
Certain thrones provide Gem Steal Chance Bonus, increasing the green zone on the spinner.

3. Gem Defense Chance
When a defending player has Gem Defense Chance upgraded, the attacker has lower chances of stealing a gem.