Series 1, Duo balls (not really a Fun Factory product, rebox item):
2 plastic balls contected with a silicone coated string from ball to ball then out into loop. Between Balls it is covered with some type of elastomer coat. other info, inner balls send out a level 6 vibe very easy.
Series 2, Duo balls (these are the real deal, Fun Factory made), Between balls is silicone only, silicone loop contains no string. Caution: one slip of the fingures on a tight exit can cause a rubber band affect on vagina. Other info, Larger inner balls send out some/no vibe feeling.