A dummy unit is created there, once the player has chosen a Hero, the dummy is killed, therefore no vision left for that area.
When unit is bought, the vision fades away (after few seconds), this is because there is no longer units there (killed).
If you don't understand the trigger, I suggest you should learn basics of triggering first, learn what is variable and so on.
NOTE: Don't mind the leaks.
That's why I said, it's no use to keep asking people to make it for you, it would be better if you learn it by yourselves.
You can look for tutorials there are many of them over on The Hive Workshop, you just need to have a strong will to learn all of these.
Slowly, you will learn, don't be lazy to read tutorials
In short, Variable is something that we can save a data to it.
You can name your variable to anything you want, as long it makes sense to what you are saving.
Variable is used for saving data and be used for later reference.
Example, now you save a Unit Variable name Caster.
50 seconds later, you want to kill the Caster, you can just;
And this trigger will refer to your Caster variable and kill it.
- Unit - Kill Caster
It is basically a feature that allows you to save data to it and refer it later.
It is indeed useful in world of programming, not only in Warcraft World Editor, but also in other programming languages.
I may seem a madman to you, but you need to start learning basic triggering first, search tutorials on this site, enjoy your stay
But we never know when the Players will pick their Heroes.maybe you can set the dummy unit's regen is -1, so that set its hp as the time you want.
But we never know when the Players will pick their Heroes.
This is indeed useful if you have a limited time of picking Heroes.
Next, learn about leaks.
This tutorial will surely help you: http://www.hiveworkshop.com/forums/triggers-scripts-269/things-leak-35124/?highlight=leaks
Leaks are a thing that will slowly slows down your game performance, making it lags if the leaks continues gradually over time.
It is important to clean leaks so that the game can lasts longer (instead if you have leaks in your map, your map can only lasts about 30 ~ 60 mins and after that, it would be total lag, drops in FPS, it depends on your leaks creation though)
That's why we should clean leaks whenever possible.