The intention of the neck relaxer is to do what it's name says ~ relax the neck. It is not intended to be put on and magically collect the horse, and if used to do so it won't work.
It will however, aid in getting the horse to relax his neck when being asked to collect, as some horses get nervous and tense or lock their neck when asked to do something that might be a little hard for them (everyone who owns an OTTB is probably nodding in agreement).
You can have all the push in the world from behind but if you have a horse that locks their neck you will not get collection. Sometimes flexing to loosen the neck helps, other times it just creates more of a "battle". There are some horses that respond better to a neck relaxer than to flexing to get their neck to soften.
However, if your horse fights it (for more than a few minutes) it probably isn't going to work for you, as you are only creating more tension and unwillingness.
Also a good tool for a horse that confermationally carries their neck/head high. If the horse is ewe necked or has a "track neck" (for most OTTBs) or has an overdeveloped underneck muscle due to carrying themselves that way for so long. It encourages the top muscles to develope as apposed to the undermuscles.