Switches are fairly simple to understand, they work like a light switch. There is an ON, and an OFF. Those are the only 2 positions. Now if you walk a room after you've been gone the lights are usually off right? Well the same principle works with maps. The switch will be in the off position when you start it. StarEdit refers to OFF as CLEAR, and ON as SET.
Now realizing that little tidbit, how about I show you some practical uses:
1. Halting Preserve Trigger.
This generally works by having that when the switch is SET, the trigger will run, however, if the switch is put into the CLEAR position, the trigger will not run:
Player 1
Always
Switch1 is SET
Create 1 Terran Marine at location "Marine Spawn"
Preserve Trigger
Now if we were to turn it off all you would have to do is:
Player 1
Player 1 commands 0 Marine Spawner
Switch1 is SET
CLEAR Switch1
Preserve Trigger
2. Separating Wait Blocks.
When you have 2 waits that you want to run at the same time there is a dilemma, you must wait for one to complete before you allow the other one to even begin.
However, the wait block only applies for that one type of player. So what we need to do is make the 2 wait times execute for 2 different players so they do not affect each other.
Kenoli showed a good example how to split up a trigger using switches (a method I've named Web Triggering). Now all of those triggers execute for 1 player, but what if you made one of them execute for a different player instead?
Let's say we want to create a zergling every two second and a hydralisk every five. Now other than using messy and complicated DCs (Death Counters), let's use our Web Trigger method.
Player 2
Always
Create 1 Zerg Zergling at location "Spawn" for Player 1
Wait 2000 milliseconds
Preserve Trigger
Player 1
Always
Switch 1 is CLEAR
Create 1 Zerg Hydralisk at location "Spawn" for Player 1
SET Switch 1
Preserve Trigger
Now look carefully at the player of this next one.
Player 2
Switch 1 is SET
Wait 5000 milliseconds
CLEAR Switch 1
Preserve Trigger
As you can see, both wait times won't affect each other, and you get the affect you desire.
If you need more help or perhaps an Example Map simply ask.
None.