Note how she doesn't let up on her biceps -- the entire time her arms remain at an angle. That's why they're so big and strong.
Specifically she keeps her arms at a right-angle at the elbow, which is the hardest angle at which to hold a flexed arm hang, even harder than the acute angle of a full chinup. I wish the camera came in for a close-up and that her arms weren't covered by long sleeves. However, there are some other videos of her on GWM where the camera comes in close as she rolls up her sleeve to fully show her flexed arm muscles. I just mentally combine that imagery with this workout video and imagine feeling her muscles as she supports her full body weight with her arms flexed at the angle of maximum generated power!