What is Retour ?
Working from your mother tongue into a foreign language.
Normally interpreters work into their mother tongue. Some interpreters know a second language well enough to be able to work into that language from their mother tongue. This is called "retour" interpreting. The French word for return is universally employed.
A small number of interpreters know their second active language so well that they are able to interpret into that language from all the languages in their language combination. These interpreters are said to be able to work in two booths. Retour interpreting is especially useful to provide relays out of less well-known languages into more widespread languages.