For an example of this, go to cbc.ca or newyorker.com, cut some text from any article on their website and paste it into a document. You will most likely notice a second line with “Read more:”… has been added with a URL to the story you have included. You will also see some uniquely identifying information that is sent to Tynt.
The best way to avoid this is block tcr.tynt.com which seems to be the only host that is referenced by the code as of writing this. This can be done by adding it to AdBlockPlus, Greasemonkey or a similar plugin. You can also link it to /etc/hosts as 127.0.0.1. I would feel comfortable blocking *.tynt.com as that seems to be their main line of business.
Another alternative is something like Tynt Blocker although we haven’t verified the integrity of this plugin, so use with caution.
If Tynt can do this to compromise your privacy, it won’t be long until these techniques are used to compromise the security of your computer, as what else could a website inject into your next copy/paste?