yes, bias is stretchy. not too bad if you sew two pieces or HSTs & one of them is on the straight grain. but if you end
up sewing two bias edges together, you can have problems,
especially if you don't pin ends. that's why you cut your outside setting triangles from a large square into quarter-square triangles. then you have the straight-grain edges on
the outside of your quilt. Cutting HSTs with the shown method
works really accurately if you first use bias strips, then cut them with a square-up ruler. method is called bias-square.
I'll check to see if there is a tutorial somewhere.