So I am trying to learn a trick which involoves culling.
I noticed that every time I cull the card the card looks like as if it disappear.
When I cull, I put my left thumb on the card before the selected card to prevent movement from the right hand won't become isolated (don't know if you know what I am saying)
also, I noticed that if I don't use my left thumb, the card is always stuck half way and can't be culled properly

Any advice?
Thanks very much

You're doing it right.
A cull that doesn't use the thumb is Kostya Kimlat's RoadRunner cull but it only works when you're culling a number of cards one after the other (not a stack) because the first card you cull is done the same way like you do now.
If you're worried about the card "disappearing" then you can do a convincing control by aligning two cards together, showing only one of them and then culling it, that way it will look like nothing happened.
Here is a video of what I just explained:
http://www.youtube.com/watch?v=iUyGuejLuuE
The guy in the video does it very bad but you get the idea

what i want is a spread cull since it is required in my trick
any advice on spread cull?
thanks very much

I'm not exactly sure what advice you need. First off this seems a bit spoilerish, but even so it sounds like you got the right idea. Just keep practicing. And I HIGHLY suggest purchasing the Road Runner Cull. Even if you never learn all the things in the video, the basics greatly improves this sleight. Also, to avoid the card seemingly "disappearing" you need to make your movements flow together with purpose. This will take some playing around and practicing.

ps In the Road Runner Cull video, Kostya goes over speed, tempo, and rhythm to make your culls look pretty. Once again, well worth the purchase.

-ArchAngel_G

thanks very much for the source^^