Here is what I use the disconnect button for:
I have an old vCenter 4 install with 10 ESXi 4.0 hosts, I have now built a vCenter 5.5 system and need to migrate to this vCenter but I cannot have any down time.
So I hit the disconnect button then add the ESXi 4.0 host into my new vCenter, then at my convenience I then vmotion of the 4.0 host onto a 5.5 host.
I hope this makes sense for you.
Simon.