Are these your first sessions? I am not a fan of the technique the decoys are using. That is also some interesting line agitation, far too close together for my liking (for molossers).
As a handler, best thing to do is have both hands on the leash. Get yourself a nice 6 foot leather, and a correction collar on some sort of tab. Looping your hand in the leash is a death sentence. You are trapping you and your dog if something happens. Get in the habit of two hands, loop in the right, with the loop around your thumb and across your palm with your fingers grasping it. This makes a lock, while allowing you to totally release the leash in a hurry if need be, as there is no way you can physically pull your thumb through your palm. Your left hand guides, lets out or reels in, and corrects the dog. If you get a correction collar on a separate tab, you can "out" your dog, reinforce it, and mean it on the first try, not cranking his neck around with the flat a few dozen times. Agitation and bite work is rough enough on the cervical vertebrae, don't make it worse...
Here is a video about proper leash technique as I have described. This is for OB, but the right hand lock technique is the same.
Your stance is good, feet wide apart, bent at the knees, with your left leading. Stay upright, do not bend at the waist, this gives the advantage to the dog and destroys the back. If you need more power, lean into your back bracing leg, in almost a sit. Pivot on you weight baring leg, and point with your lead leg. You can and should be able to complete a 360 spin in this position if you ever need to direct or follow your dog once alerted. If you place your right hand on your hip, this gives you added support. By "sitting" and pushing down with the right hand, you can control most beasts.
Here is a member, showing good form and holding a dog that outweighs her by quite a bit. This technique will provide you with superior control of your dog. The only thing I need to address in this photo is that she could have more weight on her back foot (she is moving in this photo also), but considering this dog has over 50+ pounds on her, she does a great job.