The Iain Abernethy of Taekwondo?

[Gnarlie]

I'm pretty sure I don't do it well. How should it be done?

It's a repeated combination of simultaneous arae makki and anpalmok momtong bakkat makki.

Consider the chamber positions and flight path for these basic motions, and most people do not perform them correctly, especially in the second repetition, which requires the arae makki chamber to reset to the shoulder.

A lot of people don't have the end position right, and do a movement something akin to hitting a snare drum in front of them.

This video has so much going wrong I don't know where to begin, and the scissor block is an example of what not to do:

 

[Jaeimseu]

That's a pretty literal interpretation of the movement, and I agree you're not likely use it as a paired block vs two opponents. However, there are tons and tons of other things the same movement can be used for. The same movement can be used as a disarm. Or a takedown.Or a simultaneous block and strike. All valid uses, and all using the motion taught as a scissors block in Taegeuk 7 or Palgwae 5.
Names are needed, especially when you're working with a group. But they're also limiting. Because we call it a scissors block, people don't consider that the same movement can be used as an attack.

I definitely agree that poomse movements can be assigned multiple functions (regardless of the name given). However, I've never really been interested in doing that. I'd much rather determine what I what I need and then find a technique that fits my purpose than try to find a purpose that suits a technique. Of course, I don't view poomse as a be all end all complete fighting system and don't have a desire to teach it that way. I don't have an issue with people who are into that kind of thing, but Taekwondo is interesting enough to me already without the need for much reverse engineering of poomse.

 

[Gnarlie]

I tend to view poomsae as teaching basic principles of movement. In the case of scissor block, how to move both hands simultaneously outward across the body with one high and one low. Later, when teaching some seemingly unrelated self defence principle, such as absorbing and trapping a kick, I can refer back to this movement and say 'it is similar to scissor block', and students have a movement image that they have practiced thousands of times to apply. As opposed to starting every technique with a clean slate and learning it in isolation, which is slower and does not give the same structure in learning.