bendover, the string can slip in any style of clamp, but with flying clamps there are two strings being held, so there is twice the potential for slippage as with a fixed clamp. Flying clamps can also twist. One side of the clamp is holding onto a previously tensioned string and the other onto the string now being pulled. When the tension head is released, the force is all transfered to the clamp. Since its anchor point is another string and string is flexible, the clamp can twist or draw-back. Also, there is no way to avoid pulling two strings at once on the center mains with this type of machine. This will result in lower tension on these two strings unless one compensates by raising the tension on the arm.