This article is from the The Young Ones FAQ, by Andrew Wong BritCom@audiophile.com with numerous contributions by others.
As you may (or may not) know, the illusion of moving pictures (and hence
television) is created by flashing 25 pictures per second in front of your
eyes. If these pictures consecutively show someone moving, then the person
appears to move before your very eyes...
Of course this can be abused, as in the case of subliminal advertising
(I'll get there in a second, don't worry....) It was rumoured that 1950s
films and TV shows would occasionally throw up a message in just one frame
saying "Drink Coke" or something like that. Thus the subconscious brain
would get the message, whilst the conscious brain would remain completely
unaware of this, save perhaps a surprised blink of the eye.
A couple of pictures that are completely irrelevant to the plot of The
Young Ones pops up every once in a while during the programme.
In the second season, flash-frame images included a dove in flight, a
jumping frog, a skier, a dripping tap and the "The End" caption from
Carry On Cowboy.