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Ok, but what happens? Will I get a prompt asking me if I want to do this or will it just start right then as soon as I click?

Do I get an option for an amount to divest?

No, I just tested on an account and as soon as you click it begins at the maximum amount.

It would be better it appears a confirmation message.... but if I remember right it is not the end of the World since there is an option to cancel the process afterwards... (please somebody confirm)