The problem with that sentence is that it contains a pun on words that is difficult to translate to Spanish. Let's see the sentence in its complete context:
“Why, that ain’t anything. I can’t fall; that ain’t the way it is in the book. The book says, ‘Then with one back-handed stroke he slew poor Guy of Guisborne.’ You’re to turn around and let me hit you in the back.”
So the kid is misinterpreting the expression "with one back-handed stroke", which would be translated into Spanish as "con un golpe de revés", giving it the sense of "stabbing someone in the back" instead of "stabbing someone in the direction of the back [of the hand]". As noted by a related answer I got in the English language site, the kids from the Tom Sawyer novel are speaking the lines of the Robin Hood book "from memory", so they are speaking lines of an unfamiliar version of the English language, such as "Who art thou that dares to hold such language?". So it is natural that the kids got some lines wrong.
We now face the problem of adapting, and I really mean adapting and not translating, the work into Spanish and keep the pun of words. So we could say:
—Pero eso no tiene nada que ver. Yo no puedo caer. Así no está en el libro. El libro dice: «Entonces, con una estocada de revés mató al pobre Guy de Guisborne». Tienes que ponerte al revés y dejar que te pegue en la espalda.
This sentence in Spanish may not have much sense, because what does "ponerte al revés" mean? To turn around? To be upside-down and walking on your hands? And what's more, is "estocada de revés" an expression that can be misinterpreted in Spanish? Maybe not.
Another option would be:
—Pero eso no tiene nada que ver. Yo no puedo caer. Así no está en el libro. El libro dice: «Entonces, con una estocada a mano vuelta mató al pobre Guy de Guisborne». Tienes que darte la vuelta y dejar que te pegue en la espalda.
But we face the same problem: is "estocada a mano vuelta" an expression the kid could misunderstand?
So the translator chose to keep the pun in this way:
—Pero eso no tiene nada que ver. Yo no puedo caer. Así no está en el libro. El libro dice: «Entonces, con una estocada traicionera mató al pobre Guy de Guisborne». Tienes que volverte y dejar que te pegue en la espalda.
Now "una estocada traicionera" may be understood in several ways. What does it mean? A stab that Guy of Guisborne did not see coming? A mortal stab? A stab given in a treacherous way or with a treacherous intention? The aforementioned post from the English site mentions that a thrust given with the back of the sword itself may indeed be considered treacherous. In Spanish a "stab in the back" can be easily considered a "treacherous stab", so it's easy for the kid to understand "una estocada traicionera" as "a stab in the back", even though "una estocada traicionera" does not necessarily imply that.
Remember that keeping the puns of a work is the hardest part of a translator's job, and most of the times it will be needed to adapt the text rather than translating it.
By the way, the original text cited in the Tom Sawyer book comes from the Child ballads, as compiled by Francis James Child in his book titled The English and Scottish Popular Ballads, and goes like this:
Robin thought on Our Lady deere,
And soone leapt vp againe,
And thus he came with an awkwarde stroke;
Good Sir Guy hee has slayne.
Which is translated, according to the book Baladas de Robin Hood by J. Rubén Valdés Miyares, as:
Robin pensó en Nuestra Señora loada,
y enseguida dio un salto,
y así asestó una estocada de revés;
al buen Sir Guy ha matado.
That seems to confirm that the original text indeed speaks about "estocada de revés" as I initially proposed, as awkwarde means "in the direction of the backhand" (literally: "in the wrong direction"), from awk 'the wrong way' and 'warde', 'direction of' as in forward, backward.