I tip my hat to XX for the background info. In modern times, a svengali is someone who pulls strings - a maker of pop culture successes. Sometimes the puppets cut the strings, with mixed results. It's usually a pejorative, but it shouldn't be. Here's some pop svengali links, off the top of my head: Kim Fowley, Don Kirshner, Creed Taylor, Brian Epstein, New Edition, Andrew Loog Oldham... It was Gil Evans' favorite anagram.