It looks like a variation of triangle in a square idea except it's rectangular.

Those triangles are not consistently the same size, so I'm guessing either reverse applique or maybe a cut right thru the background square randomly, then stitch the opposite color on an angle, flip, then sew back together again, creating the reverse color triangle?

Those quilts are intense...the crowd was impressive too. It fascinates me how intricate and detailed the quilts created by others can be........way beyond my imagination, or patience!

Well, I don't know what you'd call it, but what I ended up doing because I was trying to get away from any raw edges, was I put two blocks together right sides, drew my triangle, sewed on the line, then cut out the triangle, ironed it open and put a backing fabric behind that and topstitched it in. No raw edges.
There would be a definite learning curve, (ha, ha) to getting perfect points on the triangles but at least it isn't driving me crazy anymore trying to figure it out. And, I'm not sure you could do more than one on a block without distorting it too much.

Watson
(Don't mind the topstitiching...I was too lazy to take off my FMQ foot.)

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That's what I would have said too. They really have a way of setting their quilts apart from anyone else's. They seem to work magic in design and execution.

Watson! By golly, looks like you solved that mystery!

Wow beautiful quilts. I love the blackened white one also. I was going to suggest doing something like you did. Now I feel like I am going to have to go play with this. Very interesting idea. I also like the circles. I have been formulating something my mind with a bunch of circles.

I was thinking reverse applique as well. However, Ricky Tims has a method where he sews in those narrow pieces.

I remember seeing a technique similar to the one here : https://www.youtube.com/watch?v=U7JtBaNQtB4. It was a way to make an inset triangle by inserting a folded shape into a seam. Haven't been able to find the info again, but it was similar to the technique in the video.