When you have camera uptilt, your rolls start including more and more yaw. When you are flying normally, you can learn to compensate for this. But when you do snap-rolls, the camera doesn't pivot around the center of the field of view, and this can be undesirable in some circumstances. In this video, I'll show you how to compensate for camera uptilt using a technique I call cross-coordination.
This whole video is done from the perspective of, "your FPV camera has uptilt and you can use cross-coordination to cancel out the uptilt." If your camera doesn't have any uptilt, then you can play with the opposite of this. Instead of doing just rolls, add some yaw in the same direction, to cause your rolls to look as if your camera did have uptilt.