Is dental floss really effective?
The evidence shows that we don't know. Here the main issue is the lack of good evidence.
How we find how effective is? Simple: we took a group of people with similar age, similar dental status, and divide randomly to one group who will use floss and other who don't. Then, we wait for one, two or five years and compare ...
The correct use of dental floss is to use a new portion of the floss for every site. The easiest way to achieve it is through the loop or circle technique, using a new part of the loop every time.
Here is a video
Also, always use the floss before the toothbrush. If you use after, you will remove toothpaste (fluoride) deposited in the surface of the teeth.