Now you know what it is you may also think it strange when some units gain experience.

Routing troops for instance will sometimes glitter with experience upgrade.

Its very hard to get screenshots of this. Pausing the game does not stop the glittering effect wearing off. By the time you see it, zoom in close, print screen the effect has gone.

I love the effect. Not because its pretty but as a sign of success in the 'role play' nature of unit development.