I am not entirely sure if you are referring to a particular position or to the whole exercise?
Yes to pressing the lower back down, engaging just before the movement. But at the top of the movement the body is supposed to resemble a jackknife, or a ‘V’, so the torso is off the ground.
Mm yeah, maybe it should. Having these details in the description is very helpful, especially for someone new to the exercise. I only knew this detail because of other experiences I’ve had outside the app, otherwise one may not know it.