Comment by kiicia

It depends on how big/dense is your screen, high frequency message is actual text while low frequency message is decoy text. Probably so that it shows when frames are averaged together without checking actual motion vectors. It is similar effect like that image showing either young woman or old man depending on distance you are viewing it from (or other one with either Einstein or Marylin Monroe)

[edit] http://lentitaliane.it/blog/wp-content/uploads/2014/10/illus...