They have a whole process including things like counting your "umms" and "aahs" and "likes" and other verbal fillers. A good group gives you constructive criticism from others who have gone through the process themselves. I learned to pace myself better and to slow down. The speaking skills I had developed in presenting to technical audiences were not as useful for non-technical presentations and Toastmasters helped me improve there.
I learned a ton and didn't even make it through the first 10 speeches[1]. Some off the top of my head:
* Organization -- learning how to center a speech around a topic with 2-3 supporting points.
* Brevity -- the 3 min time limit is hard to adhere to, and it takes practice. Which leads to...
* Practice -- practicing giving speeches, speaking extemporaneously, standing in front of others... all incredibly valuable.
* Handling criticism -- criticism is built into the process. After you give a speech, someone else follows up with a critique. Which may seem incredibly frightening at first, but normalizing it and making it part of the process in this manner is incredibly powerful and confidence/resilience boosting. (Plus, in my experience, people are quite positive and uplifting in their critiques.)
Beyond any specific insights or ideas that I learned, I think it's the overall process that's the magic. It builds confidence that extends beyond formal public speaking and impacts your everyday interactions as well.
[1] I wish I had stuck with it longer. I moved and never found a new group to join. This thread reminds me that I should look for one again!