With this implementation you don't, but it's not hard to construct it such that there is no single frame with enough information to construct the whole message. If you screen capture there are several frames where you really can't tell what the middle letter is, but it's obvious what the message is.

The real issue is a computer can compare the images individually and combine them so if it get's 'G_G', 'GF_', and '_FG' it's going to assume 'GFG'.