This is a very common problem when the Handler approaches, the dog gets nervous in anticipation of the “Sit” Helper step back.
The first step is to always wait 5-10 seconds after coming to the dog before saying Sit. This breaks Classical Conditioning. We break the link between the arrival of the Handler and the fight coming to an end.
Another option is to go and stand next to the dog and after 5-10 seconds give the Bite command. This changes the dog’s expectations.
In the video below you see Tobias creating a link between the arrival of the Handler and the escalation of the fight. The dog does not even want to look at the Handler at the end of the session. They are doing Guarding Phase, the Handler moves and tries to distract the dog, Tobias instantly switches to agitation and the goes bark to Guarding Phase. Sometimes he does not switch to agitation but simply gives the dog a bite when the Handler approaches. It is important not to create a Behavior Chain.