the description is "Set the boolean to the input value (true or false) if mode is true, otherwise retain the last value. Return the current value of the boolean.
Go to http://zone.ni.com/devzone/devzoneweb.nsf/opendoc?openagent&B9EF354FFA6BC538862569F800816602&cat=98EEA3AF25CFCA42862568CD0066090E. I think this is what you are looking for.
What about a global (or local depending the case) inside a case structure? If you wire the case true, it will be updated if not, leave case empty and it won't change. Hope this helps