Actually they do use there thumb b y b. That's it. Its very hard but it takes lots of practice. Now another option is jump canceling with the analog stick. That's a whole nother story lol. I almost got it down.
@SUPERNEB What forward did to the sheik is indeed a jump canceled shine. You can immediatly jump out of a shine. Which is very handy in comboing opponents. He shined then pressed x or y, or he shined, than jumped up using the analog stick and than daired. The reason why it happened so quick is because you can jump out of the shine 1 frame after it's executed which is an incredibly small amount of time. There's 60 frames in 1 second.
ALso about shooting your opponents down. It depends on the percentages. If there at a ridiculously high percentage of course they will shoot down but. If they are at "reasonable" damage jump (double jump if needed) dair and than you need to fast fall down before your opponent hits the ground. To give you a better understand of this is go to practice mode and shine a peach and follow her in the air and dair her, FAST FALL and than when you hit the ground she should be above you where you can shine again. You'll be able to see it for yourself in viperboys combo video.
http://www.youtube.com/watch?v=SDYJzRABZFQ