It's really easy to reproduce.
Lowering the game speed helps if ur a noob like myself.

1. ctrl + 1 to group 2 templars
2. cast maelstrom on 1 templar
3. immediately after press 1 and mash shift + r to merge the boyz

Also works with 1 templar already in stasis as shown in the video below.

I think it has been known in Korea for awhile but haven't seen anyone talk about it outside.

Protoss may not have many ASL titles but they got style...

Firebathero showcases it 33 minutes in [timestamped]