The key new is added on top of the original harmonic key, and hangs down and off to the side.
The idea is for the key to feel like how the throat A key feels on a clarinet. The key allows you to rock into the front F without lifting your finger. Due to design, on a lot of horns it is difficult to quickly activate the harmonic key. It might be resting too high, or you might pinch your finger on the bottom, or you might just be unable to reach it.
This add-on solves those problems, and a teflon sleeve gives the piece the slipperiness you need to start playing your fast and high licks.
Of course, the drawback is that this piece is soldered on. And to attach it, I had to file the top of the existing key flat, both to expose the brass and to create a level surface for me to solder the key to. The key can easily be removed with a torch, but the flat spot will remain.