Oh my goodness ! I just watched the video. that lady waving an open rotary cutter around gave me the chills ! and not using a ruler to cut those lines ??yike, what a recipe for a really nasty accident. can you imagine your blade angling across & ruining all that fabric--let alone cutting yourself!
sorry if I rant--but cutter safety becomes more important after you see a nasty "accident".
these are bias-square HSTs with a technique first shown by
Marsha McCloskey years ago. early method is very accurate for smaller units,especialy if you have a lot of them to produce, but a little hard to understand until you do it.