The Flipper comes out underneath the hand at delivery. There must be sufficient tension in the wrist and fingers to impart a good helping of backspin or underspin. In doing so the Flipper will float on towards the batsman and land on a length fuller than he first anticipated, often leaving him caught on the back foot when he wrongly assumes it to be a pullable or a cutable ball. The back spin or underspin will cause the ball to hurry on at great pace with very little bounce, though it it is harder to achieve this on softer wickets I think. The trajectory of a smaller leg spinnner or overspinner with its dropping looping flight, bowled in a series to a batsmen will have him used to the ball dropping on a shorter length, therefore when he sees the flipper slightly shorter he wrongly assumes that it will drop and loop like a normal overspinning delivery. Shane Warne, in his earlier days had Robin Smith with a beauty, After giving him 3 or 4 wronguns with overspin, he produced a flipper on off stump. When Robin Smith went back to cut, to his dismay he found it landing fuller than he expected and skidding thru, rapping him on the pads plumb in front of off stump.
Shane Warne's Flipper is bowled with the seam coming down like a inswinger, bowling in this way disguises it and creates the tension in the wrist/fingers that I mentioned earlier. The flipper will only cause a batsmen who cannot read a legspinner out the hand any problems, Sachin Tendulkar is a classic example of a batsmen that can read them very well, I have never seen him troubled by Warne's flipper, maybe he doesn't read it, but it's just that he has such amazing eyes, reflexes and footwork that he deals with it anyway. The flipper will take much practise, though after a couple of days one can have it skidding on, though having said this it doesn't mean you will be accurate, or be able to disguise it. Just goes to show how great bowlers like Grimmett, Benaud and Warne were and are to achieve all this.