In 2006 I send a message to Alcor (Nicholas Jitkoff ) on the blacktree forum with the idea of a single modifier activation for Quicksilver.
My thinking was this…
When you press a modifier key (such as command or shift) OS X is expecting you to press another key.
You never use a modifier key by itself.
We are short of function keys etc, so I proposed that Quicksilver be activated by the single press of a modifier key. (The command key).
There was a potential problem though, it is possible that you press down a modifier key (e.g. Command) and then change your mind and let it go.
So I suggested that Quicksilver look for how long you held the modify key down, and if you hold the modify key down for under 100 milliseconds, then Quicksilver treat that as invoking Quicksilver. Because, realistically, you never just press a modify key down briefly like that.
Alcor responded fast and within a week this functionality for ‘ Modifier-only activation’ was introduced into Quicksilver!
I am not aware of anyone else using modifier only activation…
In 2012 Apple introduced dictation into OS X and the had a feature to ‘press function key twice’ to activate dictation.
Take a look at the keyboard shortcuts for dictation in the current Apple OS X system preferences…
Granted, it’s not single modifier-only activation, but it’s close.
I don’t know if they borrowed this idea of me (via Quicksilver) or they came up with it themselves but I am happy to claim the credit!