Many times, it is very subtle and can be as simple as letting another man know you exist.
No need to be explosive
As I said, if used wisely, it is truly a weapon that can save a marriage or a relationship.
In my opinion, the fact that you gf talks with another guy is not a reason to give up on the relationship.
It would be a pity.
Here is an example:
Your gf chats with a guy at a party.
Many men might feel slightly threatened by that.
Rather than withdrawing and feeling miserable, you can walk to them, look at him straight in the eyes, smile and say: "Hi, I'm Paul, Jane's boyfriend..."
Often that's all you need to get your message through and it works!
Is this still an expression of jealousy?
Maybe...
It is definitely a battle strategy that shifts the power balance in that situation ;)
http://vitalcoaching.com/jealousyformen.htm