The last frame (LVF) is the reference. The app searches earlier frames for the one that looks most like it.
Optional: drag on the last frame to compare only part of the image (ignore a moving background, a logo, a caption…).
Higher is more similar. Click the curve or a candidate to change the start frame, then compare it against the LVF.
H.264 MP4, video only (no audio). Set your player or <video loop> to repeat it.