Flying Open (with the glove side) is a problem with one's pitching mechanics that lots of people talk about, and many know is wrong, but I've never seen anybody do a good job of explaining why it happens and how it's bad.
Flying Open is a problem because it can compromise a pitcher's Timing.
In the worst case, it can lead to Early Torso Rotation.
Flying Open (with the Glove Side)
Steven Matz is an example of a pitcher who has a problem with Flying Open and whose injury problems are, in my opinion, directly attributable to flying open.
The thing to notice in the clips above is how Steven Matz starts to pull with his glove side elbow what his pitching arm is FLAT and not UP.
In the best case, that creates a serious Timing problem that I call Flat Arm Syndrome and that I believe is at the root of Steven Matz's elbow problems. In the worst case, it creates a serious problem called Early Torso Rotation that gets the shoulder, not just the elbow.
You see the same pattern in Marco Gonzales and Stephen Strasburg; the glove side pulls hard while the pitching arm is FLAT and not UP.
That isn't Stephen Strasburg's only problem, but it's the icing on the cake.
Henry Owens is another pitcher, and just one of several Red Sox pitchers, who has a problem with Flying Open (with the Glove Side) and a Timing problem as a result.
The biggest problem with the flaw of flying open (with the glove side) is that it is now being taught as something pitchers want to do. I believe that is because it can help quickly boost a pitcher's velocity, at the cost of a significantly increased risk of injury problems.
Hyun-Jin Ryu is an example of a pitcher who has developed a terrible problem with flying open.
I don't know what he's been taught, but I know the Dodgers have bought into the ideas of Driveline, and what Ryu is doing is consistent with what Driveline teaches in terms of the glove side.
It will also keep Hyun-Jin Ryu from staying healthy.