I wasnt aware and would not have thought this possible since Filemaker 11 would have to convert from the new layout formats (which it doesnt understand / know about) to the older ones.
This would have to be developed for FileMaker 11, which would not make sense for FMI to do.
Are you sure you read that 12 can be converted to 11? If so where did you read it if you dont mind me asking?
AFAIK, this is not possible.
I would also say that, strictly speaking, this is not possible.
What you can try is to use the clipboard (copy+paste) to transfer elements one by one and re-build a new fp7-database using those clips, in the proper order. You will need to make sure that no new fmp12-functionality is used, and there are various elements that can't be copy-pasted (like value lists and a few more). If the database isn't extremely complex, this shouldn't be too much work.
The one type of element that is handled by the clipboard but cannot be copy-pasted from 12 to 11 are layout objects. I think the easiest solution is to just re-create the layouts by hand in FM11.
If you have unlimited time and/or great xml skills, you could write an xslt to translate the xml clipboard data from FM12's xml2 format to FM11's xmlo. If you do, I think you'll have a great product on your hands - I'd be happy to buy it! ;-)
Data would need to be transferred using one of the formats that FM12 exports and FM11 imports.