After many weeks of practice I manage to do this quite difficult sleight but almost always it flashes under the card. What am I doing wrong ? Al Schneider doesn't says anything about position of cards and flashing. Any tips?

One way that people flash is to bend the card too much before the snap. You really don't need to bend it more than a bit to get a magical snap. That may be the problem, unless you are inadvertently lifting the coin above the card, which can be easily remidied by just paying attention.

Thank you mr Rubinstein for your willing to help. Im learning the Matrix from Al Schneider Magic book. Unfortunately in the instructions there he is not say anything about snap or bending the card. The photos also doesn't show to bend the card that cover the coin. My problem is that when I move my right hand to the right to take the coin , the coin flashes and I think that I follow the instructions correctly

Sorry for the confusion. It is my mistake . I mean the “pick up the coin” move in the beginning when you cover the coins. This is what I can not do properly .

What kind of mat are you using? I found that I needed a mat that could be compressed ( more give) to help with that move.

I can do the move but most of the times the coin flashes under the card…

Are you flashing right after you pick up the coin or afterwards?

Right after I pick the coin, when I move hand to the right, the coin flashes most of the times.
Maybe has to do with the position of cards in my hand?

So are you pulling the coin up or is the card coming down?

It is hard to correct your mistake without actually seeing where you flash. My best advice always is to film yourself, and see what you can do to fix it. Good luck!

Do you have a smartphone? Take a video clip of the sleight (audience view please) and post the clip on FB - then post a link here and folks will see what you are describing. Then we can move the discussion to "secret sessions" and discuss Al's sleight. OKay?

Simple and good idea ! here we are https://youtu.be/lv6y4IFPf2k?si=FU1QqpJDruNtD80Z

It would seems that you need to point the edge of the card facing your specator downward, to hide your fingers. You also need to take into account the position of your body and hand, as well as the camera. You might want to stand up to make the tilting of the card edge downward a bit easier. For me, this should correct the issue.
Also, is the camera at the eye level of your spectators, or below? If below, you could be getting an angle that your audience wouldn't see

Here are videos of two guys who do the sleight well
Since we are still in open sessions - can't discuss details - but you can see how they handle the problem you described.

https://www.google.com/search?client=fir......rld#ip=1

Mr Rubinstein both of your suggestions are to the point. Camera is a bit lower than spectator’s eye. Also the card must go a bit down. This hides it. Thanks a lot for your help.