No it would be the poll pressure more. Using the lower rein would give leverage, but as there is a curb chain that leverage will not be lost, if you see what I mean? The leverage and the curb chain will encourage the horse to give with the jaw and bend at th poll (Hopefully!)Unless it is a jointed pelham and then a certain amount is lost through the "break" of the bit. TBH can't see the point of a jointed pelham myself though.
Horse will probably back off it a bit tbh. A ported straight bar is agreeable to a lot of horses. In my experience it would bring the head down as well. What is it ridden in at the mo?
Personally I am not a fan of continental gags, although have used them occasionally.
Pelhams are a good bit to try and useful for reschooling too!
The port you are referring to is probably a cambridge dinky one, which will do nothing than over a weeny bit of tongue relief.
Now you could get a coscoquero or hanoverian they have bigger ports design to work on the roof of the mouth when the curb is used. Different kettle of fish altogether and only for riders with good hands!