Ninja Effect... How?

I want to know how people do "ninja effects", you know, the images left behind because of super fast speed.

Generally they use a temporary dummy unit with the same model and decrease it's opacity.

I would the following:

- Make a periodic event (let's say every 0.5/1 second) that creates a dummy unit exactly like the ninja with a decreased alpha vertex coloring (as Naakaloh said);
- The new unit should have no movement speed, no attack and the locust ability (so it won't be selected/damaged).
- Give to the new unit a timed life (for example 1 second or so);
- Set the new unit's animation equal to the current animation of the ninja;
- Set the new unit's animation speed equal to 0 (so it will be like "frozen");

I hope it helps.