FaceSwap is designed to automatically detect and swap faces in images stored on your device. The app works in many regards, but its facial recognition technology feels dated and often inaccurate, and there is no way to know which images it will work with and which ones it won't. As a result, there is quite a bit of trial and error, and because the process is automated, you can't change how the images are swapped.
When you start FaceSwap for the first time, you can choose an image from your photo library or take a new one. Like most face-swapping apps, it really only works well if you have two subjects directly facing the camera in even lighting. Side shots, different shaped faces, and awkward poses often lead to distorted images or the app will fail completely, not allowing you to do the swap. This is a limitation of the technology, but without an option to see when it will happen or the ability to manually mark faces that similar apps offer, it can be frustrating to get the right image to work.
When the app does work, however, the effect is fun and the images are rendered quickly. It manages to blur lines enough that the faces blend nicely into the other person. The effect is at times jarring, occasionally hilarious, and generally well presented, but again only when the original photo is the right size, distance, and shape, and in the right lighting. If you are interested in testing a free face-swapping app, though, this is a decent one to start with as long as you're willing to find the right images to work with.