The main meaning of backwards is "towards the direction that is opposite to the one in which you are facing".
back is a bit more complicated because, though it can mean backwards, its main meaning is "towards a previous place or condition". If your leg is in already in front of you, it is unambiguous, because the two meanings, backwards and "towards a previous place or position" mean the same.
If your leg is in its normal position directly beneath your body, "move your leg back" is still unambiguous, because it can only mean backwards.
If your leg is behind you, there is a possibility for confusion, because the backwards meaning indicates moving the foot further back, whereas "towards a previous place or position" would mean returning the foot to its normal position directly beneath your body.
That said, many people whose first language is not English, and also some native English speakers, will not understand the distinction. My advice when teaching motor skills is to try the shortest way of saying it. If that works, stick with it: if it doesn't work, try something different next time.
I would also recommend using the word move rather than bring, which usually means "to take or carry something".
As an aside, I have noticed that some Russian-speaking yoga teachers say "leg" when they mean "foot". I guess that the same word can be used for both in Russian.