So what exactly is back-translation and how is the method used different from that used in 'normal' translation?
In a normal translation process, a document is translated by a translator and then reviewed by a second linguist (the editor). Both the translator and the editor work from the original source document. It is the job of the editor to look out for errors or missing text in the translation.
The process of back-translation, however, is very different. In this process, rather than involving an editor, a second translator translates the target document back to the language of the original source file, i.e. if the document is originally translated from French into English, the back-translator will translate from English into French. The second translator works from the translation only and never consults the original source file. The original source file and the back-translation are then compared (or reconciled) in order to validate the accuracy of the translation and check for missing text or other translation errors. It is important that the person carrying out the reconciliation is aware that the two versions will never be exactly the same. The aim is to look for major discrepancies between the two versions. If there are major discrepancies, the original translator must be consulted and the translation amended accordingly.
As far as the translator is concerned, it is important to be aware when working on a back-translation that the source document needs to be followed more closely than it would be when producing a 'normal', idiomatic and flowing translation. A back-translation is not supposed to be a polished text in the source language. Since the point of a back-translation is to flag up errors in the original translation, it is particularly important that the translator does not 'improve' errors or weak areas as they normally would. The sentences used in the back-translation may, however, follow the normal rules of the source language grammar.
So, translating and back-translating are very different. When translating, clear and natural forms are used; when back-translating, more literal forms are favoured as a method for showing up errors or discrepancies in the original translation.