We had one- you do have to show the dog the boundary line. Ours had an audible warning and if the dog continued to approach the line it would get shocked. We could adjust the audible warning to sound at different distances from the shock line.
Now while it worked great we had this old farm dog named Sadie. She was quite intelligent. We ( even the company Invisible Fence) could not figure out why we were changing batteries so often on Sadie's Collar. The company gave us a new collar, we tried switching collars with the other dog. Made no difference whatever collar Sadie wore- the battery lasted 2 weeks. That dog loved to roam.
So I started watching her. When she'd get a new battery she would walk the entire fence 3 feet away from the line- the exact distance the warning was set for. After a week- she made a break for it!! Dang dog figured out that when she could no longer hear the warning it was safe to run!! Invisible Fencd said they had never heard of a dog doing that. I'm sure other dogs have found the weak point by now!!😃 4951