Horizontal Vs. Vertical Blade Play

Side-to-side and up-and-down play in folding knives: what's the difference, how do they occur, and can they be fixed?
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[@TheDemocultis]

Rock solid info. Great video!

[@passwurd7987]

Over time, I learned that with Vertical Blade Play, the best way to test it is to have a slight push back and forth at around 50% of the blade for most locks, for most frame locks you can going to 75%. Locks that pass this are very unlikely to fail and spine wack tests, which while a good test, can result in damage to the lock of your knife in some cases. I think going and putting torque on the tip doesn't really show the strength correctly. Framelocks are also far less likely to fail in practical use because squeezing the handle strengthens the lock.
The only time this test doesn't really work is with Shark and Super locks. With my Axis/RAM locks, the lockup can be stellar, but I think this is because the angle of the lockface on the blade is shallow. The superlock and shark have steeper angles or a pinning system in the superlock, and because of this, the lock can rock a bit, while still being stronger at a certain point. Both my AD20.5 and Tactile Chupacabra had lock-rock with very light swaying at 50% of the blade because of the angle/pinning inside the lock, but they'd both survive spine wacks without issue. Still, for me, the feeling of the blade rocking gives me a little fear, so I don't keep these types of knives for long.
For horizontal I just test by pointing the blade away from me, squeezing the back of the blade around 5/6 of the way up and putting torque just before you feel force in my finger joint. Curious how other people value their play, but for me this is my way to go for sure.