Think of swing structures as a house, and internal structures as the stuff inside the house. Just like the furniture lives inside the house, the internal structure lives inside the swing structure. And if you consider price as something that can be broken down into smaller bits (like a fractal), then the internal structure is just the swing structure seen at a smaller scale or time.
To enable the feature simply head over to the indicator settings.
When you are inside the SMC settings - locate the "Market structure" section and make sure to check the internal structure box.
This feature will set the rule of the break of the structure. If you want to have it on close the candle needs to close below or above the low or high. See here an example:
The fake out detection feature will filter out weak structure breaks. You can compare this as a anti-liquidation feature. Enabling this will give you stronger structure levels.