Many effects in magic -- particularly close-up magic -- rely on what magicians refer to as "a thumb tip." A thumb tip is, simply, the tip of the magician's thumb if it has been prepared by trimming the nail to a certain length and coating the very end with clear nail polish.
A thumb prepared in this way is useful for a great many things. For example:
The polish on the thumb tip helps ensure the smooth insertion and removal of items from beneath the thumbnail. Some magicians worry that the shiny polish will seem suspicious to the audience, but as an experiment a magician painted his nail with bright red polish and nobody in the audience seemed to even notice.
Note: A wise magician never stores flash powder in a thumb tip, particularly if the magician smokes.