SLP here. It's thought by some in the field that stuttering may be at least in part a mismatch between your speech production and your speech monitoring system (auditory feedback). The DAF app (I've never seen an app! Just a clunky handheld thingy) works, we think, by ameliorating this errored system or maybe through some sort of habit-breaking (cognitive dysfluency, the psych term not the speech term) wherein the brain has to actively work again (and watch the security cameras, as it were) and ends up fixing things. I had one supervisor who said to be careful because sometimes the effects do not last... Your brain can become immune to the delay, as it were. She claimed that training people in proper use of the DAF device could prevent this. I haven't worked with them much but they're an exciting option. http://onlinelibrary.wiley.com/doi/10.1080/13682820310001616976/full (one relevant article; there are many on Scholar)