Topsy Tail is actually the name brand product from scünci, but it has become known also as the name of the hairstyle. Mine are actually a knock-off brand from Hong Kong that I bought on eBay for $5.
To do the hairstyle, begin with a simple low ponytail.
Insert the topsy tailer into the ponytail about the hairband. Push it all the way through until the point comes out underneath the ponytail.
Pull the topsy tailer down, making sure that the hair doesn't fall out of the loop. The hair will be pulled through the hole and make a twist.
Separate the pony into two sections and gently pull them apart to make the hairband slide up a little higher to tighten it all up.
There are more ways than one to do this. If you don't have the actual topsy tailer, you can use a craft needle (plastic or metal) like this: