There's been a lot of talk about the slider that Nova added in the middle of the 2011 season, but few talk about his changed mechanics. His hand position is the biggest difference.
In 2011, Nova brought his hands above his head during the windup. Sometime in 2012, he changed this by keeping his hands level to his chest. This does a number of things, but most importantly it allows him to keep his head steady and maintain eye contact with the catcher. When he had his hands over his head, Nova was forced to drop his head, thus causing him to lose focus of home plate and slouch prior to his stride.
Since 2010, Nova's BB% has dropped from 9.2%, to 8.1%, to 7.5%. Keeping his hands to his chest may be one factor in his improved walk rates.
David Phelps made the same adjustment as Nova. Although he does keep his hands slightly higher, Phelps drastically improved his posture at the beginning of his windup, and his head stays remarkably level. Since 2012 was his first year in the majors, I'm not sure what kind of a effect this had on his control.
In both situations, the hand positions are reminiscent of CC Sabathia's, and I suspect that the Yankees' veteran pitcher made some sort of contribution.
Betances has faced his share of control problems, and there were also changes made to his hand position. Although it's not as dramatic as Nova's or Phelp's, the right-hander now keeps his hands level with his chest, instead of starting with his hands at his waist.
The second change he's made is with his back foot. Once he drives off his back leg he now keeps his foot much closer to the mound. I suspect this has something to do with maintaining balance, and again help him with his control.