I don't think you can increase thresholds , but I do think you can decrease them , by creating an expectation of some anxiety producing event .
A dog that is prone to be stressed , stresses , is a worry wort .
Prey drive I believe can be enhanced by making it successful and fun . Chase and movement are exciting .
So by the very opening of your question , do you have a dog that is tentative , cautious , has lower stress coping ability and less prey drive than you wish.
I don't know anything about your trainer personally but from the sounds of the advice I would be giving them the pink slip. See you later .
Training devices do not replace training communication.
Instead of laying it on when the dog "needs fine tuning" why not encourage her into the right position. Be more inventive , creative , in your training which should not be a boring drill of repetititon. Not in the initial stages .
Just imagine a child learning to print. You wouldn't slap away the pencil and tear up the page if the first lines are wiggly and varied in size ? You would praise the effort , appreciate the steps .
The dog will understand what you want out of it if you recognize the positive attempt , praise it . (the attempt).
Same as you would praise the child attempting to master penmenship.
Your dog is learning helplessness . It can't figure out how not to get a correction , even getting corrections at left turns. This dog is going to hate left turns. What do you think will happen when the dog workds off lead ?
First left turn , (no correction possible) , dog runs off , relieved.
Alpha roll in herding ? Terrible . Alpha is not dominant - there is such a thing as an alpha leader . A leader inspires following , not by supression or dominance but by setting an example .
Reclaim your dog .
Carmspack Working German Shepherd Dogs