Higher level twizzles tend to end backwards (but can end forward) and consist of around 8+ double three turns on the same leg in a straight line but with vertually no edge inbetween if that makes sense. Often in competition (dance) they usually do two sets of these after each other using both the feet and alternating positions. One of my coaches once said it's basically a backspin that travels but should not look like that
. I think in the lower levels they teach a twizzle in more of a triple three turn way which after completing it on one leg you switch to the alternative.
Double three turns are a forward 3 turn immediately followed by a back three turn all on the same leg. These can be done both inside & outside.
Edit: Clarified a little!