At the beginning of this guide, it was stated that not all fins are going to be ideal for freediving. One would think that any old pair would do, which is why this question is probably so common. However, this could not be more wrong and the primary reason why is the difference between different types of diving. For snorkeling and freediving, in particular, the former is predicated more on the surface and near reefs. Meanwhile, the latter allows you to dive as far as your body can allow before you need air.
Due to the engineering of freediving fins, they are optimized for rapid descent and will not be effective while you are hovering next to the surface. Plus, with snorkeling versions, they will allow for advanced movement and provide you with more control. Basically, stick to the type of diving that they are designed for.