[Zanderax]

When an opponent scout your base by running a unit into the front, he gets an idea of what you have by what kills the unit and the small amount of vision that the unit has before it dies. This is a lot of scouting information and can win and lose games. Fortunately what he see doesn't always have to be accurate. Giving an opponent false scouting information can be good but give them the wrong type or amount of wrong information can instigate an attack that will lose you the game.

There are 3 main types of false information you can give, they are listed in difficulty to execute.

1) 'Smaller'
2) 'Tech'
3) 'Bigger'

'Smaller' involves not only not allowing the opponent to see your army but also making him believe that you aren't hiding your army but simply don't have it. If he senses an opportunity then he will do one of two things. Attack or Macro. If he chooses macro then you continue to play your game uninterrupted but if he chooses attack then you better be ready to defend against all he has. Making your army look smaller is only good if you are ready to deal with their entire army.

'Tech' involves either hiding your tech tree branches or creating fake ones. This is harder to pull off than hiding your army because it means that you have to build mostly useless buildings and units. However this can be worth it's cost. Consider this scenario: you are a Zerg player who runs a zergling into a Protoss players base around the 9 minute mark. You see a stargate, it gets chronobosted and your zergling dies. You build a spore crawler at each of your bases, you build 2 new queens, you may even go further and get a hydralisk den. The protoss player used 150 Minerals, 150 Gas and 25 energy from his Nexus to force you to do all of that. Alternativly you can use halucination to

‘Bigger’ is very hard to pull off and generaly is not worth the effort. Though good placement of your units and halucination you can make your opponent believe that he is facing an army that is bigger than you have. Used properly you can stop a big push and keep macroing or catch up on your army.
Also be sure to be aware of these things that can happen when you scout yourself.