Spinner's pointer lands randomly, so basically all depends on your luck. The default parameter is 50/50 for 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 your reach the totem, the better the chances of stealing a gem.

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

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