Well, my question is easy, i just keep failing when i bring everything up in a line.
I cant get the momentum, they said the motion depends on the hand which move up and down. but when i do that it goes jerking the yoyo instead. and my strings on my non throw hand cant keep steady on its place. the thing whole is jerking instead of doing a triangle moving back and forth.
any tips ???
Practice! 
It helps to have a stable yoyo with a forgiving catch zone when first learning. Although I can do it with my Lyn Fury now, when I first learned it, I could only do it with my DM2 and I would fail every time I took out the Lyn Fury.
For me, a good next step was to get into position (split bottom then front underpass), and then “rock” the yoyo forward and then back before engaging the throw hand “up and down” motion. Even just rocking it back and forth will “tell” your finger what rhythm to expect.
Although tutorials make it look like the up and down motion creates the boinging motion from scratch, this is not true. The momentum has to be there first, and then the up and down motion supports and augments the back-and-forth.
In other words, don’t rely on the up-and-down to create the back and forth. It won’t work. And in my case, creating the back and forth by swinging/rocking the yoyo to start it up, worked well.
However, that’s not the place to stop. The momentum we’re talking about is best created by the motion of getting into position itself. The yoyo should gather momentum as you mount it in the split bottom mount, and you should carry this through to the underpass. For me, this means that coming into position, my yoyo is already going “forward” (it is back as the underpass is near completion, ready to move forward as the fingers are in place), and then I immediately move into the down-and-up of the throw hand in order to support and build on this existing momentum.
It’s good to understand why the throw hand moves up and down. Just like with Eli Hops, you are simply accomodating the laws of physics. Picture a yoyo swinging with no intervention… the yoyo will move in an arc, right? But with Boing-e-boing, the yoyo moves in a straight line back and forth, so:
As it moves either forward or back, the throw hand finger moves down to give it the necessary slack. As it comes back to the middle, the throw hand finger moves up to its neutral position, or where it would be if the boing-e-boing wasn’t in motion (plus a little bit extra to add more energy and keep it going). Thus, you need to go up-and-down twice for every full back and forth!
It’s “good” to understand this, but it’s not necessary… in truth, what will probably happen is that you will hit it once and the feel for it will just “click”… and you’ll start to get better and more consistent with it.
Good luck!
try to kepts your hands aligned in a straight position , make sure the string is dead center and then add alil bit of momentum .
All though this is a very very very difficult trick . This one needs alot of practice in order just to get your hand memorized the motion , then after that is just 3 words : Practice makes perfect !
Don’t worry , you’re not the only having trouble with this , i remember when i still had trouble with it , i still having trouble with it but the the more you practice the more it easier to do 
I agree that the trick is very easily learned with certain yo-yos, I also found that it worked very well with the Dark Magic II, Burnside and the like. As stated above, the trick came together for me, when I used one fluid motion from the split bottom mount into the up and down motion that makes the yo-yo swing in and out. Do not try to line everything up and start the momentum from there, use the momentum from getting into the mount to start the yo-yo moving as your throw hand moves up and down. I said all that differently than above, because sometimes when things are rephrased, it can give you a better idea.
Also, it sounds like you do not realize that you are moving your non-throw hand slightly while focusing on the movement of your throw hand and the yo-yo. The difficulty of the trick, for me, had a lot to do with the multitasking. Making sure the strings are lined up, keeping a steady non-throw hand, and resisting the temptation to move the throw hand in the direction the yo-yo is moving. It is bizarre, in my mind, that my hand is moving up and down vertically, but the yo-yo is moving in and out horizontally. You have to get that multitasking done in your head as far as focusing your eyes on the “full picture” of not moving that non-throw hand, keeping things lined up, and doing the proper motion with the throw hand. It takes practice and time, and it is frustrating. If you have a Dark Magic II to learn on, use it, it is a great yo-yo to learn any trick on. It worked for me too, and it boings well. Once you get comfortable on that yo-yo, you can move to any yo-yo.
I was having the same problem and found that it was jerking because I was moving my hand up and down too quickly. Try slowing the movement down until you get the feel of it. Hope this helps.