Scheimpflug can be tricky to understand. To increase your confusion, try reading Merklinger who explains details you haven't yet imagined.
Here is a thought experiment, which is almost practical.
First choose a suitable subject with detailed objects at the top and bottom of the screen. Zero all camera movements.
Focus on a close object, which will be at the top of the screen. Instruct your (imaginary) assistant to hold the lens panel firmly at the top, so that it does not move and that part of the image stays in focus.
Loosen the tilt mechanism.
Now re-focus on a distant object, at the bottom of the screen, which will move the front standard backwards as you focus. The top of the panel, held firmly by your trusty assistant, will stay where it is and the bottom will be swung backwards by the focusing movement. You have now focused on two different points and the lensboard will be tilted. It will be obvious that the plane of focus has tilted.
You can do the same with the rear standard if you have rear focusing and also with swings if your camera has them.
I have found that most people can respond quite easily to the idea of "focusing on two different places at the same time" rather than being asked to visualise invisible, imaginary lines drawn in the air. The idea that the three planes "open like a book" seems to follow naturally as I wave my hands about. You can even point to the place on the ground where the spine of the imaginary book lies.
After this, but not before, the Scheimpflug principle comes as a welcome summary of what they've seen in practice.
I have to add that holding one part of the camera rigidly in place while you focus is much trickier than I'm describing here, but it does involve the enquirer directly in the process.
I've never explained the wedge-shaped limits of the depth of field at this point.
A note on swinging both standards the same way. If your camera had no shifts, you might use this in the situation where you are photographing a reflective object, like a framed picture, and want to avoid your own reflection in the glass. It will work for any situation where you cannot place the camera centrally in front of the subject. There might be a tree or a statue of Queen Victoria at the centre point. It creates the effect of using sideways shift. The alternative is to put the camera on its side and use front rise sideways, but this can give problems with a heavy camera.