If I remeber correctly, you have to set a trigger similiar to this:
Conditions:
Switch 1 is Set
Player 1 brings at least 1 Terran Marine to Location 1
Actions:
Turn off vison for Player 1 at Location 2
Preserve Trigger
This isn't down to the point accurate, but I haven't made a map or even attempted to make one in 6 months. So I'll give a breakdown on how it works. When you bring the specified unit to the designated location you have to run the AI script that unallies vision for yourself (Ex. Player 1 brings Marine to Location 1, the AI script must be set to turn vision off for Player 1). Of course you know that with vision off you can't do anything so you need to make an area with map revealers to place units so that you can turn vision back on, heres an example:
Conditons:
Player 1 brings at least 1 Terran Ghost to Location 3
Switch 1 is Cleared
Actions:
Turn Vision on for Player 1 at Location 2
Preserve Trigger
The switches help sort things out and I don't really think you have to use them to stack and switches are kinda tricky and confusing, so I recomend you stay away from them unless you have experience making maps.