Ms. Jackie Cochran wrote:

"If I ever get hooked on collection I am going to make sure that there is a curb bit in the horse’s mouth when I ask for it."

Collection does not occur because of the type of bit used, because in the 'old' days, dressage horses were schooled in reined cavessons to do all the gaits, and movements including collection; before the bit was introduced.

We modern day riders seem to be hooked on the need of a bit in order to achieve high caliber dressage and collection. 

Collection is the result of impeding forward.