Rails 3 and Devise. Kill current session

I got an Rails 3 app that uses Devise. I am just wondering how I can "kill" the current session?

This works but I do not know what it does

Devise.sign_out_all_scopes ? sign_out : sign_out(resource_name)

This does not work

current_user = nil

This does not work either

session[:current_user] = nil