Hopefully the implication is that you won't get cut in half like the pantomime horse they show!
Funnily enough I was recently sent some pictures from subways around the world and how they advertise the same point of not getting yourself stuck in closing doors.
Firstly, here's one from Italy of a man who appears to be lying in between the closing doors:
However, it's warning us not to lean on the doors as you might fall out of them if they suddenly open.
Then we have one from Japan with a cat with its tail trapped in the doors:
Finally another cartoon animal from France encountering some pain with a trapped hand:
So you have been warned those closing doors are dangerous particularly if you happen to be a cartoon character or a pantomime horse.
Unfortunately I'm one of those terrible people who often legs it for the Tube at the last minute as the doors are closing, block them with some part of my body, use Herculean force to prize them open and then slip myself into the carriage, as the doors slam behind me, much to the amusement or tuts of the rest of the passengers. This is often followed by stern telling off from the driver who says "Please do not block the doors while the train is trying to leave" or something similar. Is that just me?