Hope this helps you visualize how to use the tightening key thingy...
In the first pic, I'm using the key thingy as a lever to turn the cable as I hold the needle tip stationary. Think of the letter "T". If you grip the horizontal bar as handles, you can turn the vertical bar very easily. But if you try to hold a "l" and turn it, your hands will slip and you won't get a very good grip. The tightening key acts like the horizontal bar on a "T". Get it?
The second picture shows the non-slip piece I use to get a better grip on the needle tip.
I use both the grip and the key together. One hand holding the needle tip with the grip, the other hand turning the key... although I couldn't take a picture of me using both because, unfortunately, I only have two hands. Using this method, I get a very tight connection and haven't had a tip come loose yet. I even have to use the same method to unscrew the tips when I'm done!