Submitting text only versions of web pages to the search engines in an effort to gain high rankings and once the desired positions are achieved, swapping the search engine friendly text page for a content page designed for humans.
 
Code swapping, referred to as bait and switch, is also sometimes used in an attempt to keep others from learning how the page ranked well, although it’s not really useful or necessary today.
 
The major downside of code swapping is that a search engine may revisit the web site at any time, and if it indexes the "real" page the positions will likely drop. Or worse, you are discovered and your pages are dropped from the index. Ouch!
 
Side Note: Fortunately, you don’t need to code swap to do well with the search engines. A highly qualified search engine specialist or Internet consultant can secure excellent ranking results for you without the need for controversial tactics like code swapping or web page cloaking.