Hi Melissa,
The way this works is that if the Send an Email? checkbox is selected, any of the Save buttons will send the email indicated in the Send an Email section on the right.
Unfortunately there isn't currently a way to automate sending out the email, but what you can do is check the Send an Email? box, enter in all the information for your email, then uncheck the Send an Email? box and click Save or Save & Back. This will save the information you entered for the email, but won't actually send it out. Don't worry about the Send an Email section disappearing after you uncheck the box, if you navigate back into the Activity you will see the information is still there. Optionally, you could customize the screen to have the Send an Email section visible on the screen at all times.
Hope this helps.
- Justin