“Leaking forward” is generally a feeling of flying open a bit with the front shoulder. Now, this can happen because his body is not working in sequence and his lower body is going too quick for his upper body.
The most common correction on this site is to “get the hips going quicker.” I completely agree with this assessment, but it should be clarified for some of the younger guys here. While we want to be quick to the plate, and get our leg up and down as fast and efficiently as possible, there is still a such thing as “too quick.” The upper body needs to be slightly behind the lower body so that it is in a position to throw at the right time. This is probably what Jenks was feeling: he was rushing his delivery and opening the front shoulder a bit.
When Jenks says that he was dropping his elbow it is because he felt like he was pushing the ball to the plate rather than staying through it and throwing on more of a downhill plane. Now “downhill plane” does not mean throwing straight over the top, it’s simply making the path of the hand more efficient after external rotation: bringing it straighter to the plate rather than out and around. I think even sidearm guys can throw “downhill.”
He probably felt as if he was dropping his elbow because the front shoulder was flying a bit, forcing his upper body to move more side to side.
So this now brings up the point that when we analyze pitching injuries/mechanics, watching one or two videos, or looking at random still images does not tell the entire story. Jenks may have had the best mechanics in the world based off one delivery, but if he is not repeating this delivery every time, then he is stressing his body in a different way each time he pitches. That’s one of the reasons why injuries occur: guys cannot repeat their delivery consistently.