How did I thumb-scroll by letter in Contacts?!

[mstevens]

A short while ago I was scrolling through my massive list of contacts (in the default Contacts app) using the thumb wheel. Normally I don't do this since it takes too long to get to the part of the alphabet I want. At some point in the process, a small box popped up containing a single letter, and as I turned the thumb wheel the letter changed. When I pressed the wheel the app went to that letter in my contact list much as if I had clicked one of the letters in the index at the top of the screen.

For the life of me, I can't figure out how I made that happen and can't reproduce it. Any theories out there?