The onTouch property is a place to put script that you want to run only when the group is touched.
Example 
Edit
You have two boxes:
- One of them is connected to a Piston, which gets a variable: getVar("variable")
- The other box, has the script: if frame>=1; setVar("variable",1);end
Now, if the box that contains the script is touched, the variable will be set to 1.
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