Taken together the cards show a simple message or picture.

Sometimes the cards have a different colour on each side (say white and green), so the picture can be changed when the cards are flipped.

So far so simple and predictable.

So let's keep those two sided cards but instead use the crowd as a binary state cellular automaton (like Conway's 'Game of Life').

Some members of the crowd would be told to show the white side of their card at the start, everyone else showing green.

When a whistle is blown everyone flips their card based on their neighbours' cards colours, using the same rules as Life. And so on.

With the right initial settings (to make good use of blocks, oscillators, still-lifes and gliders) a crude animation could be made, such as a player scoring a goal.

It wouldn't look perfect and of course some spectators would get the transition rule wrong, which might lead to the 'ball' not reaching the 'goal', but that might just add to the effect.

The crowd on the other side of the stadium could cheer/groan depending on the success of the shot.