Apologies. A "double" is a double contact. Usually with your hands. In volleyball, the rule has always been that you can only make one, single contact with the ball when you play it. There are some variations on how that's been called over the years. For instance, when receiving a ball from across the net (first touch), if the ball hits your arms and bounces off of your chest, a while back they started to call that a single contact because it was continuous. Wasn't always the case. There was a time that any time you played a ball with your hands on a serve, it was pretty much always a double.
Starting this season, they removed that call from how the setter handles the ball (second contact). That has always been where you see that double contact called the most. I liken it to holding in football. It isn't always the easiest call to make and it can be an absolute killer call when there's an exciting play happening. They have given more and more leeway on that call for years and they decided to just eliminate it unless you are sending it over the net (third contact) or it's just blatantly intentional. The result has been some wild looking sets from weird angles that are keeping the ball in play. They did this in order to make the game more exciting. I initially hated the change but I've completely changed my mind and am eager to see the rule change at the HS and club level. Again, it's not a bad rule but the application of it is objectively inconsistent, at best, and can easily swing the game against a team that has an inexperienced setter.
Setting has always been something of an art. Need really soft hands and outstanding aim to make perfect sets. It's really tough to do that when you're out of position or having to make balls travel in directions opposite of how your body is moving. For Pitt, this means it's easier to get the ball to a position where a player can attack it even when the first pass isn't perfect or you're all over the court during a long rally. Now, aim and strength is more important which Fairbanks has. It's allowing Pitt to be more attack oriented which is really good for what Fisher tries to do.