See, I don't think they are small. I think the difference is huge - 3-5 mph - if you bend your knee too soon, or keep it bent too long.
And the 3X only matters for the back leg, the front does not need to be extended.
Again, I am no pitching expert, I am just trying to learn this stuff.
OP, sorry to get off on a tangent here.
I believe that too often people try to attribute the extension you talk about to particular movements when it's more of a combination. I focus more on the segmental separation that I believe is the engine behind quick upper body rotation and velocity. So, for me, I don't care as much how you get there - just that you do. Now, I fully realize that others - you and Eric Cressey for example - find the "how" more important than I do. Although, to an extent I do buy some of what Cressey believes concerning overuse of the upper torso to achieve the effect being possibly counterproductive.