1) The fajin must start at a specific point: I found that if I made the fajin to long for the target (i.e. springing out of the leg too soon), I lost a lot in terms of speed and accuracy.
2) The fajin must accelerate to a specific point, like throwing a frisbee: if I didn't have this mind intent, I either had to clench the handle to decelerate the blade after cutting or carry way past the target. In either case, a crafy duifang could easily take advantage of the resulting pause or opening.
3) Swing speed really tests one's body mechanics: I think that unless one practices the form at near full-speed, it's hard to appreciate how much the momentum of a cutting swing can throw oneself off balance. Many times I noticed that when my mechanics weren't good enough or I didn't have the proper focus, my shoulders twisted, the blade carried too far, and my fajin was much less effective.
4) Attention to blade angle: Cuts that were newer to me, such as the liao cut, were not as accurate because I wasn't confident about the blade direction and angle of attack. The nicked corner of Laoshi's stand can attest to this

Lots of food for thought. And not surprisingly, all things that Laoshi has scolded us about during sword class

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