The detache' stroke is used when you have a series of equal value notes at a medium or fast tempo and you want them to sound smooth, legato.
Detache' is easy to describe but it takes times to develop and refine.
Sometimes the detache' stroke is taught by asking the student to fling the down bows away from the perpendicular path. The danger is that many students can never get the bow to straighten out, even after a couple of years! Their exaggerated motion can become ingrained, possibly because they had achieved a balance doing it.
I've created a series of exercises to help you understand and develop a reliable detache' that sounds good and can ultimately be used for fast passages.
(I'm working on a 4 part video series on detache')