Let’s look at how to use of the rear arm. There are two main possibilities:
Extend your arm toward the opponent:
Extend the rear arm towards his face so he can’t see well anymore. If you have enough power, you can even use this movement to hit him in the face.
It makes it hard for him to counter with a technique. For just a second, he only sees your glove or fist and can’t see you launch your leg kick.
Tighten down the arm to hinder him if he tries to move in on you or fire a technique of his own. This is a bit more advanced but it can work real well to stop him from landing shots on you.
Put your rear arm across his body and sweep him to the floor. This is a little bit less a power kick meant to injure him than in other cases. But if you time it well, he lands hard on the floor and it breaks his concentration a bit.
Swing it back and down:
Simultaneously swing it back and down. This stabilizes the leg kick a bit and makes it easier to stay in balance.
It’s the easiest way to move your back arm when you throw a leg kick. Because of that, you can focus more on generating power and landing your shin as hard as you can..
This uncovers your face though. Be aware that this leaves you vulnerable, especially to punches. To compensate, lean back a bit,time the leg kick when he won’t be able to move in on you your leg kick or as you step back again.
Another option is to hunch the rear shoulder up and into the side of your face. This gives you some extra shielding against a potential counter punch.
The most important thing here is to know why you use the one method or the other. They both have positive and negative aspects. If you’re not sure what to do, experiment with a training partner or capture your sparring sessions on video to see what works best.
For more information on the rear arm, visit this link for more detail on how to do a leg kick.