Play games
Choose the game that interests you and play itfor free.
Complete the quests
Every game has some quests for which you will receive Soul Gems.
Receive rewards
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
I tried it 3 times with 100 on green/white/green and burned 3x100 SGs. From what I see you get 2x the amount you put in when you pick green or white and win. So it appears to be simply a way to burn your hard earned SG, like in a casino where the house wins in the long run. What are your experiences with it so far?