The send log button is only available in beta builds. It is located in the About tab. From v3.7.3beta you must have verbose log enabled to be able to send a log.

The log includes device and ROM information. Please include a short description of the problem and/or a mention of the thread where the issue is being discussed.

The log is extremely useful to detect what’s going wrong if you think JuiceDefender is not behaving as expected. In that case you might want to check the entire log yourself, to do this you can either send the log email to yourself, or, after tapping on send log, discard the email and open the /sdcard/jdlog.txt file.