Hi
I expected "fn Key Down" action will keep fn Key pressed down similar to CMD Key Down / Ctrl Key Down. However, it doesn't keep pressed down at all.
For example, I configured the sequence as Double fn Key to activate "fn Key Down" action. Then I have very custom shortcuts by using "fn + ..." (such as "fn + J" to simulate key "arrow left").
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I am seeing a similar problem. I am trying to use the Keyboard Modifier Actions of Option Down and Option Up, as well as Control Down and Control Up. However, assigning these actions to Keyboard Shortcuts have no effect.
Mac Pro, late 2019
macOS 10.15.5 with the Supplemental Update.
Better Touch Tool 3.389
Thank you very much for testing. Given that the Actions work for you, I continued to investigate, and found the cause. I was using the Option key in my Key Sequence (which is the Key being acted upon by the Action.). When I removed it from the Key Sequence, the problem did not occur.
I also tried your issue, assigning FN FN to the Action fn Key Down. As with you, it did not work.
Regards and good luck.