One of the Preferences for Update Index is “Automatically.” So what does this mean? Last night I clicked Update Now, and this morning I set it to update Automatically. When will DSE in fact update the index? Is it based on an interval of time? An operation? A condition? Something else?
The Help says “Choose how often you would like DEVONsphere Express to update its index from Manually to Automatically,” which is utterly unclear to me. 
Ditto. This was frustrating to me as well. The “Help” only made me angry in that it further obfuscated the situation as you point out.
Note to DT: If you give me an option for a choice, tell me the implications of my decision! Does Automatic mean the program indexes when more resources are available? Immediately after every save or content change? Randomly?
Specify for clarity not obfuscate for frustration!
Pretty basic stuff.
Automatic updating means that DEVONsphere Express will update its index whenever files are added to the indexed folders, are modified, or deleted. The update occurs within a few minutes delay to balance the impact on the performance of your computer.