Jacknives lower back

A doubt about lower back in jacknives.

Wouldn’t it must to be executed with lower back always in the ground?

I ask that because in other exercises the app always recommends put lower back in the ground and at jacknives doesn’t recommend.

A simple search at internet, all tutorials recommends never do it with arch back.

Other thing, the app video shows clarity that the guy engages yours lower back earlier than start execute.

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.

Particular position. Problem is the app doesn’t recommend put lower back in 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.