English Translation
Turn the knife in the wound.
Rubbing salt in the wound hurts more, but reduces the chance of infection.
Turning the knife in the wound is completely bad.
Let us hope it is metaphorical.
“Retourner le couteau dans une plaie” (which, apparently has quite a few variants), does very much not hava anything good to it. Remuer le fer dans la plaie, if the metal is hot enough…
But I digress, what I was here to say is that I feel that a “to” is missing in the english sentence, isn’t it? I know there are situations in which the bare infinitive is used, but this sentence doesn’t seems to be one of such situations (the wiktionnary does add it in its entry). So, in my opinion, this is to be interpreted as a more direct sentence, rather than as the idiom, and so, as an imperative instead of an infinitive. Which would make the french translation: “Retourne/retournez le couteau dans la plaie”.
After some digging around, I think I found the original tatoeba sentence. From the languages I can understand, I think there is a division in whether the sentence is to be interpreted as the idiom or as a regular sentence, although there is a comment pointing out that the verb is in imperative…