FYI, the Details tag only works if there’s an empty line above it. For example:
words
[details=Summary]This text will NOT be hidden[/details]
words
Summary
This text will NOT be hidden
It looks like it works in the post preview, but when you actually post it messes up. On the other hand:
words
[details=Summary]This text WILL be hidden[/details]
words
Summary
This text WILL be hidden
If you have any more questions/issues, check out this thread.