Strengthen function helps your Pirates to become stronger, it is another way to level up a certain Pirate much faster than the usual grinding. However, there's an exchange if you would want to do this process.
1. To start, tap on the Training Grounds in your Hideout.
2. Tap on "Strengthen".
3. Tap first on the pirate that you want to strengthen.
4. Then tap on the pirate(s) that will be sacrificed or Strengthening Item(s) that will be consumed.
5. Then tap "Level Up".
Take note that the first pirate that you select in Strengthen function will be the one to receive the benefits of it, the other selected pirates will be sacrificed and removed from your roster so be careful always!
If you use a pirate to sacrifice that's the same with the one you would like to strengthen, not only it will increase the experience but also unlocks the a swabbie slot.