The defensive trick is to not leave enough room for you to get started in the first place. Unfortunately, far too many people do as you describe...let you in to avoid the accident...then post and whine about it..
But I can still get in. The cars don't line up exactly right behind each other so there is a spot where one car is farther to the left then the other.
THAT is what you call an opening. Stick the nose of your car right up into the rear bumper of the car that is just a few inches more to the right then the one behind him.
The one behind him MUST let you in. They've got no where to go because I've put my car in your path, you can't turn or move to the right or you'll h it me. If you want to effectively block you must be even with the left side of the car in front of you or an inch or two to the right. If you're a little bit to the left, then I'm in.
This is why the diagram has it correct. To properly wedge in there you've got to have a good angle to block somebody and get in. You can't be close, have to make a steep angle of doucheness to be successful.