This sequence represents the introduction of a second small triangle in the piece.
The idea is to use both small triangles (2x T1) to visually show a square , a parallelogram and a medium triangle (T2). All constituting the tangram.
Once these pieces are on stage we can bring the last 2 geometrical shapes: 2 large triangles (T3).
The first part shows the first exploration with only a square introduced. The second section (1:15) contains also a parallelogram and a medium triangle (T2).