When you touch soundgreen:
Event 1 - are you touching soundgreen? - yes - swap them
Event 2 - are you touching soundred? - yes (you are now) - swap them back
End result is nothing changes.
'System: Wait 0 seconds' will postpone the following actions until the end of the event sheet so soundred won't get swapped in before the check for touch on soundred.
<img src="https://dl.dropbox.com/u/8367729/construct/pics/toggleButton.png" border="0" />