This was also a great tip for me. I saw someone do it when I was onsite at an enduser.
Everyone who works with the key’s in NAV tables hassled with this, I am sure.
When you hit F6 on a key you get the field list. This is only usefull when you add a key on one field. What you want is to go to the form that allowes you to add more than one keyfield.
This (seems to be) able to achieve with shift+F2.