Imagine going to the general store and buying so much stuff that eventually you’re awarded by getting something back as well. Think of it as a gift. This is how this feature works. You just play the game as you would normally do under any other circumstances and eventually you are going to fulfill the requirements for getting a few free bonus rounds. This isn’t guaranteed though. You may activate free spins two times in a row or go 100 rounds without doing so even one time.
The mode is usually activated by getting a certain number of a specific symbol. Most slot machines have a symbol tagged as the scatter. Landing or 3 scatter symbols triggers the mode awarding you some free spins. In other cases, they might be awarded if you fill a bar meter or gather a number of artifacts, etc.
Generally speaking, bonus games come in many forms featuring various other smaller mechanics. Other than being offered several free spins, you may also get an increased number of wilds to appear, high paying symbols only, sticky/expanding/shifting symbols or reels, and so on. Also, in many cases, there is a way to get even more free games by landing on a number of scatter symbols or fulfilling another requirement.
Even though the advantages are plenty, increased profits/no cost, etc., there are some disadvantages too. These are mostly risk-centered as some games may offer you the ability to choose a free spins mode. These can vary in nature and volatility, meaning that as the risk gets higher the risk, the bigger the potential payout gets as well.
The different forms of free spins are: