Objective C because that was the most viable option for native Mac development at the time, and I needed access to keyboard events system wide for it to be useful.

What it did was listen for keydown/up events on the spacebar, and swap the system keyboard layout to a horizontally reversed qwerty layout while the spacebar was held for some (arbitrary based on what felt right to me) duration. When released it would restore the normal qwerty layout.

This made it relatively cognitively easy to treat my right hand as a reversed “left” hand and use my touch typing muscle memory while I was unable to type with my actual left hand. If I remember correctly, I took a typing speed test and it was ~70% my normal two handed touch typing speed, which was good enough to keep working while I recovered.