From what would appear to be a single bundle of yarn, the user takes the 2 ends and then plies them back into the spinning wheel. If the yarn is wrapped properly around their hand, they can then literally create 2 plies of yarn from a single source. Those shepherds in the Andes mountains sure were clever to invent this system many years ago.
There are a number of limitations and issues when using your hand to perform this function, however.
Often times, the yarn gets very tight around the user's fingers and it becomes uncomfortable. Spinners often cite the fact that their fingers turn blue from the lack of circulation.
The user cannot easily jettison the yarn that is wrapped around their hand in case they (1) want to take a break, (2) answer the phone, (3) change the baby, etc. They have to take the time to ply all of the yarn back into the spinning wheel. This can be very inconvenient.
Other plying tools which do not have a base, can be put down on a flat surface while the user attends to these other tasks, however, the "bracelet" of yarn can then accidentally fall off of the handle which can be catastrophic.
Yet some other tools which can be safely set down, are often times too large which causes other constraints for the user.