A Beginner Level Introduction to Quasi Connectivity/BUD Powering | Minecraft - Java
I didn't know this when I made the video, but the reason this exists is because the developer re-used the code for the door, which is 2 tall, on the piston and dispenser. I can't remember who I learned this from.
I forgot to mention that some blocks don't give updates when they change states. To be precise, they give state changes (things that observes can detect) but not block updates. Examples of these are trapdoors. It's not a big deal but it really sells the fact that this block update stuff is quirky. Thanks u/H3rlittl3t0y on reddit for reminding me.
If you want a visualization of this updating phenomenon, here's a small demonstration by Space Walker:
• Neighbor Update Visualizer test