Hero Wars Mobile - Evolution Booster trick explained

The Evolution Booster is an item you can use to evolve a hero to the next evolution (for example from 5 stars to 6 stars). You can get them from the Guild War competiton vendor for CSW trophies.
Did you know you can use this to avoid spending gold?
And did you know that if you have enough soul stones, you can get refunded all the CSW trophies?