Correction of English

Question

English text had an issue: "explaining it's working"?? - not correct.  You would say perhaps, "explaining how the gym worked, or functioned (in general - thus the rules of the gym, or how to use the gym), or explaining how a particular machine worked.

Accepted Answer

Hola Gerard
That bit of text is in a hint where we say that literally it would be "explaining its working" (explicando su funcionamiento), but the actual text in the English translation offered is saying "where there was an instructor explaining how it worked."
This type of note is sometimes found in writing challenges to help a bit. 
I hope this clarified it.
Saludos
Inma