Back-Translation, how do it generate the translation?

[tall cairn]

So, I'm currently looking into how back translation is done.

I was told to go on and check on this paper by Sennrich et al https://aclanthology.org/P16-1009.pdf

But, I do not understand how the synthetic translation is obtained. It seems that the paper only talks about that back-translation helps in NMT, but doesn't go on to say how it work.

I've always assumed that they used another already pretrained NMT model and then get the synthetic translation like that, but now I am confused. Can someone please help me? x.x

[royal marsh]

You can use any translation model or even just google translate