Hi Ivan,
I'll try to answer your questions, but remember this is the training method I use, it could be, that your dog needs another aproach...
You ask me, why don't let the dog jump around ?
It's important that if you want a good biting dog that he should only be busy with biting. It's the same as for humans, if you want to work concentrated on something you need your mind to be empty and work one thing at the time.
To get a dog biting hard and good you need to prevent stress.
You ask me; why not let the dog hugg my leg ?
You want the dog to have good grip, good grip means a hard bite and the other way around. To achieve this you don't allow the dog to have any support on his front legs, biting less hard means loosing grip ! The dog doesn't want to lose!
In this way you'll learn him to bite harder !

You ask me how to remain standing with a back attack on the leg ?
It's a feeling thing...
When you know that a dog attack's your leg try to lean forward and make big steps, really important never stretch your legs completely, you might injure yourself !
We look at what the dog finds easy leg or shoulder, but there is one advantage of the back attack on the leg, the bicycle attack will also be on the leg and this a problem with a lot of dogs.
I hope I answered your questions !
Greets,
Martijn Eijbersen