Mine has two levers; not all of these keys do. The right-hand one is moved to the left and makes a simple contact closure. The left-hand, moved to the left, does the exact same thing. So why's the other one there? I don't know.
The fun begins when you move the left-hand lever to the right. That's where the semiautomatic action starts to make the "dits" (usually written as "dots") for you. Like most other bug keys, it uses a weighted pendulum and a flat spring or reed to bounce back and forth; but the way this key shoves the pendulum into motion is madly complex, a square ascending spiral involving three levers, two pivots and a roller! I've tried to trace it:
And if you weren't confused enough, here's a back view that shows the around-two-corners lever a little more clearly.
I first saw one of these keys in 1996 at a railroad museum in French Lick, Indiana; it was in a display case and I was unable to get a clear enough look at it to figure out how it worked. I've wanted one ever since.
Photo links are from this page at Tom Perera's online museum, where a key-fancier can easily spend hours. I was able to find a few more examples via a websearch as well.