This is really just a technique to convert the provided Google AdWords Conversion Tracking Code into something more a little more useful.
Take the conversion code provided, usually something like:
And convert it into this:
Notice that in the second block of code above, I've taken the source of the image which was provided by Google in the first block of code's noscript tag.
That's it! with this your Google AdWords conversion no longer has to be a landing page, it can be a button click event, or whatever else you want it to be.