To me it looks like - “disable synchronization”, which in turn sounds like suspend DC’s ability to sync messages, it sounds the same as - disable background synchronization on Android (and users knows what does it mean), and if I activate it, then synchronization would be disabled, that’s what it tells.
As I wrote before, “Download unseen message only” IMHO describes what you want more clearly.
Please, don’t think about me as a monster, I simply working with a lot of real different users for really long time and simply share with you my experience.
General users don’t like or afraid or simply lazy to explore new/complex stuff… unfortunately
That’s not about my opinion, it’s about statistics.
Yes, that what Im trying to explain you.
I bet that 3/4 of users would think about it literally - “disable message sync”, which means completely disable message synchronization, but in fact it would still download unseen messages which will rise the question, - “why messages still coming ?”
It’s just a wording and I think it is better to describe action exactly what it should do.
I afraid we don’t understand each other 
I’m not an enemy to your proposal at all and I really would be glad if people in situations like yours would have a choice to communicate while using expensive or unreliable communication channels.
What I don’t want - is to attempt to broke common sense.
Could you try please to understand me, you talking about “disable message synchronization”.
Now, assume you are non technical user and you having the only one instance of DeltaChat on your phone and you found in settings option - “disable message synchronization”.
First thing that comes to mind in this case - it’s suspend synchronization with IMAP server since it’s the only one party used in communication.
Wording is matter!
“Download unseen message only” != “disable message synchronization”
You obviously overreacted with unknown reason to me… which is sad, I’m trying to help actually.
My idea - isn’t to make an app with one single button, but to have UI intuitively understandable for all range of users.
If an option would be called “Download unseen message only” it’s IMO understandable,
but in this case, it will be very convenient to have access to a button on primary/frontend UI that allows
to download all/read/skipped messages.