The feature allows the find/replace dialog to auto-fill the search field with whatever your currently selected text is. This behavior is the default right now in Visual Studio 2012. Here's how to turn it off.
- Go to Tools > Options
- In the left tree under Environment, select Find and Replace
- Uncheck the checkbox for "Automatically populate Find What with text from editor.
This is a simple setting, but I found myself looking for it since I have never had the need for it and it is annoying when you are running the same search multiple times with minor changes.