Only the great magicians can perform big acts in live shows and thousands of audience. In Britain’s Got Talent, many magicians and illusionists have performed and most of them succeeded. Their secrets are never revealed but some keen watchers cracked their secrets. But I believe that the success of their magic acts is the fruit of their years of practicing and doing the same act many times.
This is why they have perfected it and delivered it to the audience very effectively. They are really great at directing the attention of the audience to one of their minor tricks. And while the audience is hooked in that minor trick, the magicians and their accomplices are busy in completing the major trick that they will reveal at the end of their act.