i've done it a few time but i can't explain how it happen, just destroying my key until it go through, what's the condition to cross the line?

In order to line clip, you have to press flip on the second frame after initially flipping off the ground (you can do them from the ceiling too, but this never comes up in the base game, only custom levels). The older way to do line clips, and the less consistent way, was what it sounds like you're doing: just mash the flip button until you make it through. An (in my opinion) easier and more consistent way to do line clips is to buffer them. Flip off the ground with a very short press, very briefly let go, and then hold the flip key until you get through the gravity line. In other words, you're looking to press flip for a frame, release for a frame, and then hold flip again through the next frame. This will be a very quick motion, but you can't go too fast, or else the game will simply register that as the button being held the entire time instead.

Try that, and see if you find it any easier.