It took me forever, but I think I figured out (more or less) how you did this :-).
Each side of the block has a triangle for half and strips for the other half (with the triangle on one side of the block opposite the strips on the other half). So you lay out your square of batting with a triangle right side up covering half of the block on the front side (going about 1/2" beyond the diagonal). Use a pin to temporarily hold it in place.
Flip the block over. Lay a triangle right side up on the back side, but diagonally opposite (not behind) the front triangle. Pin in place.
Let's complete the back side first (it doesn't matter). Lay a strip across the center, right sides together with the triangle and matching up the edges. Stitch across the diagonal through all layers (this will catch two pieces of fabric on the back side, the batting, and the edge of the triangle on the other side). Flip the strip open, press it flat, and then lay out another strip on top of the first (RST), matching raw edges, and sew 1/4" from raw edge. Continue to lay out and sew strips to that side until complete. Flip the block over and do the same to the other side.
Use your favorite method (mentioned in other posts) to sew the block together (with sashing).
I haven't done this, BTW, just guessing...am curious and will have to try it out.
Would appreciate anyone's comments and suggestions.